Top 80+ Docker Interview Questions & Answers

Docker has transformed how applications are developed and deployed by introducing a seamless way to package and run software. It has become an essential part of DevOps practices, enabling teams to work with greater flexibility and speed. By using containers, developers can ensure their applications run consistently across different environments.

With its widespread adoption, understanding Docker has become a vital skill for software engineers, DevOps professionals, and system administrators. It supports the development of scalable, portable, and lightweight applications, making it a favourite in the tech world. As more companies adopt cloud-based solutions, Docker’s relevance continues to grow.

In this blog, we’ll cover 80+ Docker interview questions, categorised into basic, intermediate, and advanced levels. This will ensure you’re well-prepared for interviews, regardless of your experience.

Basic Docker Interview Questions and Answers

1.   What is Docker?

Docker is a platform that allows developers to package applications into containers. These containers include everything needed to run the software, ensuring consistency across different environments.

2.   What is a DockerFile?

A Dockerfile is a text file that contains instructions to build a Docker image. It specifies the base image, commands to set up the environment, and how to run the application.

3.   Name and Explain the Components of Docker.

Docker consists of several key components:

• Docker Engine: The core application for building and running containers.
• Docker Images: Read-only templates used to create containers.
• Docker Containers: Running instances of Docker images.
• Docker Hub: A registry for storing and sharing Docker images.
• Docker Compose: A tool for defining and managing multi-container applications.

4.   List the Most Used Commands of Docker.

Here are some commonly used Docker commands:

• docker run: Starts a new container.
• docker build: Builds an image from a Dockerfile.
• docker pull: Downloads an image from a registry.
• docker push: Uploads an image to a registry.
• docker ps: Lists running containers.
• docker stop: Stops a running container.
• docker rm: Removes a container.
• docker rmi: Removes an image.
• docker logs: Fetches logs from a container.
• docker exec: Runs a command inside a running container.

5.   What are the Features of Docker?

Docker offers several features:

• Containerization: Encapsulates applications in containers.
• Portability: Runs consistently across various environments.
• Scalability: Easily scales applications horizontally.
• Efficiency: Lightweight compared to traditional virtual machines.
• Isolation: Separates applications for better security.
• Version Control: Manages image versions effectively.

6.   What are the Pros and Cons of Docker?

Pros:

• Simplifies application deployment.
• Enhances scalability and flexibility.
• Reduces resource usage compared to VMs.
• Facilitates continuous integration and delivery.

Cons:

• Learning curve for new users.
• Potential security vulnerabilities.
• Limited GUI tools.
• Storage management can be complex.

Must read: Kubernetes Architecture: The Ultimate Guide

7.   How Do You Create a Docker Container from a Docker Image?

To create a Docker container from an image:

• Pull the desired image:

docker pull image_name

• Run the container:

docker run [options] image_name

• Example:

docker run -d -p 80:80 nginx

8.   Name and Explain the State of a Docker Container.

Docker containers can be in various states:

• Created: Container is created but not running.
• Running: Container is actively executing.
• Paused: Container execution is temporarily halted.
• Stopped: Container has been stopped after running.
• Exited: Container has finished execution and exited.

9.   What is Docker Hub?

Docker Hub is a cloud-based registry service for Docker images. It allows users to:

• Store and share images.
• Discover official images.
• Collaborate on projects.
• Automate image builds.

It serves as a central repository for developers to access and distribute container images.

10.  What Platforms Does Docker Run On?

Docker supports multiple platforms, including:

• Windows: Docker Desktop for Windows.
• macOS: Docker Desktop for Mac.
• Linux: Various distributions like Ubuntu, CentOS, and Debian.
• Cloud Services: AWS, Azure, Google Cloud Platform.

This wide support ensures Docker can be used in diverse environments.

11.  Explain the difference between Docker and Virtualization.

Aspect	Docker (Containerization)	Virtualization (VMs)
Architecture	Shares the host OS kernel among containers.	Each VM runs its own OS on a hypervisor.
Resource Usage	Lightweight; uses fewer resources.	Resource-intensive; each VM needs separate resources.
Startup Time	Starts in seconds due to minimal overhead.	Takes minutes to boot a full OS.
Portability	High; containers run consistently across environments.	Limited; dependent on hypervisor and OS.
Isolation	Provides process-level isolation.	Offers complete isolation with separate OS.
Use Case	Best for microservices and cloud-native apps.	Suitable for running different OS environments.
Performance	Better performance due to minimal overhead.	Slower performance because of additional OS layers.
Management	Easier to manage and deploy with Docker tools.	Requires complex hypervisor management.

12.  What Command Can You Run to Export a Docker Image As an Archive?

Use the docker save command.

Syntax:

docker save -o [archive_name].tar [image_name]

Example:

docker save -o myimage.tar ubuntu:latest

This command creates a tar archive of the specified Docker image, which can be transferred or backed up.

13. What Command Can Be Run to Import a Pre-Exported Docker Image Into Another Docker Host?

Use the docker load command.

Syntax:

docker load -i [archive_name].tar

Example:

docker load -i myimage.tar

This command imports the Docker image from the tar archive into the local Docker registry, making it available for use.

Must Read: Docker and Kubernetes: What’s the Difference?

14.  Can a Paused Container Be Removed From Docker?

Yes, a paused container can be removed.

Steps:

• Unpause the container:

docker unpause [container_id]

• Remove the container:

docker rm [container_id]

• Alternatively: Force removal without unpausing:

docker rm -f [container_id]

Using the -f flag forces the removal of the container immediately.

15.  How Do You Get the Number Of Containers Running, Paused, and Stopped?

Use the docker ps command with filters.

Running Containers:

docker ps -q | wc -l

Paused Containers:

docker ps -q –filter “status=paused” | wc -l

Stopped Containers:

docker ps -a -q –filter “status=exited” | wc -l

These commands count containers based on their current state.

16. How to Start, Stop, and Kill a Container?

Start a Container: Restarts a stopped container.

docker start [container_id]

Stop a Container: Gracefully stops a running container.

docker stop [container_id]

Kill a Container:  Immediately terminates a container.

docker kill [container_id]

17.  Can You Use JSON Instead of YAML for Docker Compose?

No, Docker Compose primarily uses YAML for its configuration files.

Reason:

• Readability: YAML is more human-readable and easier to write for configurations.
• Compatibility: Docker Compose does not support JSON format for docker-compose.yml.

Recommendation: Use YAML for defining Docker Compose configurations to ensure compatibility and ease of use.

18. How Do You Build a Dockerfile?

Use the docker build command.

Steps:

• Navigate to the directory containing the Dockerfile:

cd path/to/Dockerfile

• Run the build command:

docker build -t [image_name]:[tag]

• Example:

docker build -t myapp:latest .

This command reads the Dockerfile in the current directory and builds the Docker image with the specified name and tag.

Must Read: An Introduction to Kubernetes And Containers

19.  What Command Do You Use to Push a New Image to the Docker Registry?

Use the docker push command.

Syntax:

docker push [repository_name]/[image_name]:[tag]

Example:

docker push myrepo/myapp:latest

Steps:

• Login to Docker Hub:

docker login

• Push the image:

docker push myrepo/myapp:latest

Ensure the image is correctly tagged with the repository before pushing.

20. Compare the Use of ENTRYPOINT and RUN Commands in a Dockerfile.

Aspect	ENTRYPOINT	RUN
Purpose	Defines the main executable to run when a container starts.	Executes commands during the image build process.
Timing	Runtime	Build time
Overriding	Can be overridden with –entrypoint flag.	Cannot be overridden at runtime.
Usage	Ensures the container always runs a specific command.	Used to install packages, set up environment.
Syntax	ENTRYPOINT [“executable”, “param”]	RUN command

21. What is Docker Engine?

Docker Engine is the core component of Docker responsible for creating and managing containers.

Components:

• Daemon (dockerd): Manages Docker objects like images, containers, networks.
• CLI (docker command): Interface for users to interact with Docker.
• REST API: Allows communication between Docker components.

Function: Handles container lifecycle, image management, and networking, enabling developers to build, ship, and run applications seamlessly.

22. How Do You Access a Running Container?

Use the docker exec command.

Syntax:

docker exec -it [container_id] [command]

Example: To open a bash shell inside a container:

docker exec -it container bash

Alternative: Use docker attach to connect to the main process, but docker exec is preferred for running additional commands.

23. When a Container Exists, Is It Possible for You to Lose Data?

Yes, data can be lost if not properly managed.

Scenarios:

• Non-persistent storage: Data inside the container is lost when removed.
• Improper volume usage: Not using volumes or bind mounts leads to data loss.

Prevention:

• Use Docker volumes or bind mounts to persist data outside the container lifecycle.
• Regularly back up important data to ensure its safety.

24. Is There a Limit on How Many Containers You Can Run in Docker?

Docker itself does not impose a strict limit on the number of containers.

Factors Affecting Limits:

• System Resources: CPU, memory, and storage capacity.
• Host Performance: More containers require more management.
• Network Constraints: Availability of ports and networking resources.

Best Practices:

• Monitor resource usage.
• Use orchestration tools like Kubernetes or Docker Swarm to manage large-scale deployments efficiently.

25. What is the Difference Between Docker Logs and Application Logs Inside a Container?

Aspect	Docker Logs	Application Logs
Source	Generated by Docker daemon for container events.	Generated by the application running inside the container.
Access Method	Accessed using docker logs [container_id].	Accessed within the container’s filesystem or forwarded to external systems.
Content	Includes stdout and stderr outputs from the container’s main process.	Detailed application-specific events, errors, and informational messages.
Management	Managed by Docker’s logging drivers.	Managed by the application’s logging configuration.
Use Cases	Troubleshooting container-level issues.	Monitoring application behavior and performance.

26. How Will You Use Docker for Multiple Application Environments?

Docker effectively manages multiple environments (development, testing, production).

Approaches:

• Separate Docker Compose files: Define different configurations for each environment.
• Environment Variables: Use .env files to customise settings.
• Consistent Images: Ensure the same Docker images are used across all environments.
• Orchestration Tools: Utilise Docker Swarm or Kubernetes for managing deployments.

This ensures applications behave consistently across different stages, reducing deployment issues.

27. How Do You Scale Docker Containers Horizontally?

Scale Docker containers by increasing the number of container instances.

Methods:

• Docker Compose:

docker-compose up –scale [service]=[number]

• Example:

docker-compose up –scale web=3

• Docker Swarm:

docker service scale [service_name]=[number]

• Example:

docker service scale my_web=5

• Kubernetes: Use deployments and replicas to manage scaling.

Benefits:

• Improved application availability.
• Load balancing across instances.

28. What is the Purpose of Volumes in Docker?

Volumes are used to persist data generated by and used by Docker containers.

Purpose:

• Data Persistence: Keeps data intact even if the container is removed.
• Sharing Data: Allows multiple containers to access the same data.
• Isolation: Separates data from the container’s filesystem for better management.

Usage:

• Store databases, configuration files, or user-generated content outside the container lifecycle.

29. What are the Differences Between Docker Networking and Traditional Networking?

Aspect	Docker Networking	Traditional Networking
Isolation	Uses namespaces and virtual networks for isolation.	Physical and virtual networks with limited isolation.
Scalability	Easily scalable with container orchestration tools.	Scaling often requires additional hardware or complex configurations.
Flexibility	Dynamic network configurations and on-the-fly changes.	Static configurations with manual updates.
Automation	Highly automated through Docker commands and APIs.	Manual setup and management are more common.
Integration	Seamlessly integrates with container ecosystems.	Typically independent of container technologies.

30. Where Are Docker Volumes Stored?

Docker volumes are stored in a specific directory on the host filesystem.

Default Location:

• Linux: /var/lib/docker/volumes/
• Windows: C:\ProgramData\Docker\volumes\
• macOS: Managed by Docker Desktop, stored within the VM.

Note: Volumes can be managed and inspected using Docker commands, and their exact path can vary based on Docker configuration.

31. How Far Do Docker Containers Scale?

Docker containers can scale significantly, limited primarily by the host system’s resources.

Scalability Factors:

• Host Capacity: CPU, memory, and storage.
• Network Bandwidth: Adequate for increased traffic.
• Orchestration Tools: Kubernetes, Docker Swarm help manage large-scale deployments.
• Application Design: Stateless applications scale more easily.

Example: Large-scale web services can run hundreds or thousands of container instances across multiple hosts to handle high demand efficiently.

pipeline {

agent { docker { image 'node:14' } }

stages {

stage('Build') {

steps {

sh 'npm install'

}

}

}

}

Advanced Docker Interview Questions for Experienced

62.  Describe the Lifecycle of a Docker Container.

The Docker container lifecycle consists of several states:

• Created: The container is created but not running.
• Starting: The container is in the process of starting.
• Running: The container is actively executing.
• Paused: Execution is temporarily halted.
• Stopped: The container has stopped running.
• Deleted: The container is removed from the system.

Understanding these states helps in effectively managing container operations and troubleshooting issues.

63.  What are the Differences Between Docker Community Edition (CE) and Docker Enterprise Edition (EE)?

Feature	Docker CE	Docker EE
Target Users	Individual developers and small teams.	Large organisations and enterprises.
Support	Community support via forums and documentation.	Professional support with SLA guarantees.
Features	Basic containerization and orchestration tools.	Advanced security, management, and orchestration.
Cost	Free and open-source.	Subscription-based with additional enterprise features.
Updates	Frequent updates and releases.	Stable releases with extended support periods.

64. What Is the Purpose of the “docker checkpoint” Command?

The docker checkpoint command is used to create and manage checkpoints of a running container.

Purpose:

• State Persistence: Saves the current state of a container, allowing it to be restored later.
• Migration: Facilitates moving containers between hosts without losing their state.
• Backup: Enables backing up the state of containers for disaster recovery.

Example Usage:

docker checkpoint create my_container checkpoint1

docker start –checkpoint=checkpoint1 my_container

This feature leverages CRIU (Checkpoint/Restore In Userspace) to capture and restore container states.

65.  Can We Use JSON Instead Of YAML While Developing a Docker-Compose File in Docker?

No, Docker Compose primarily uses YAML for its configuration files.

Reasons:

• Readability: YAML is more human-readable and easier to write for configurations.
• Support: Docker Compose does not support JSON format for docker-compose.yml.
• Community Standards: The ecosystem and tooling around Docker Compose are optimised for YAML.
• Recommendation: Use YAML for Docker Compose files to ensure compatibility and take advantage of Docker’s features seamlessly.

66. How Will You Ensure Container 1 Runs Before Container 2 While Using docker-compose?

Use the depends_on directive in the docker-compose.yml file.

Example:

version: ‘3’

services:

service1:

image: service1_image

service2:

image: service2_image

depends_on:

– service1

Explanation:

• depends_on: Ensures that service1 starts before service2.
• Note: It does not wait for service1 to be “ready”; additional health checks might be necessary for complete dependency management.

This setup helps in orchestrating service startup order effectively.

67.  Write a Simple Shell Script to Reclaim the Space Used Up by Docker

#!/bin/bash

# Remove stopped containers

docker container prune -f

# Remove unused images

docker image prune -a -f

# Remove unused networks

docker network prune -f

# Remove unused volumes

docker volume prune -f

# Remove all unused data

docker system prune -a -f –volumes

echo “Docker space reclaimed successfully.”

Usage:

Save the script as cleanup_docker.sh.

Make it executable:

chmod +x cleanup_docker.sh

Run the script:

./cleanup_docker.sh

This script automates the cleanup of unused Docker resources, freeing up disk space.

68.  What is the Difference Between Daemon Logging and Container Logging?

Aspect	Daemon Logging	Container Logging
Scope	Logs Docker daemon events.	Logs application events within containers.
Purpose	Monitors Docker engine behavior.	Tracks container output or errors.
Configuration	Managed in /etc/docker/daemon.json.	Configurable per container.
Examples	Start/stop events, resource allocation.	Application logs, system errors.

69.  What is the “null” Network Driver and What is Its Use?

The null network driver disables networking for a Docker container.

Use Cases:

• Isolated Environments: When a container does not require network access.
• Security: Enhances security by eliminating network exposure.
• Testing: Useful for testing container behavior without network dependencies.

Example Usage:

docker run –network=null my_image

This ensures the container operates without any network interfaces, providing a completely isolated environment.

70. How is Multi-Host Networking Achieved in Docker?

Multi-host networking in Docker is achieved using overlay networks.

Steps:

• Initialise Swarm Mode:

docker swarm init

• Create an Overlay Network:

docker network create -d overlay my_overlay

• Deploy Services to the Overlay Network:

docker service create –name my_service –network my_overlay my_image

This setup is essential for distributed applications requiring inter-container communication across hosts.

71. What All Drivers Are Available in Pre R1.9 Release of Docker Engine?

Before Docker Engine version 1.9, the primary network drivers included:

• Bridge: Default network for standalone containers, enabling communication on the same host.
• Host: Removes network isolation, using the host’s network stack directly.
• None: Disables networking for the container.
• Overlay: Introduced later, for multi-host networking.
• Macvlan: Assigns a MAC address to a container, making it appear as a physical device on the network.

These drivers provided basic networking functionalities to connect containers effectively.

72. Compare Host Network and Bridge Network in Docker.

Aspect	Host Network	Bridge Network
Definition	Shares the host’s networking namespace.	Creates an isolated network for containers.
Isolation	No network isolation; containers use the host’s network.	Provides network isolation between containers.
Performance	Better performance due to direct access.	Slight overhead due to network translation.
Use Case	High-performance applications needing direct host access.	Default for most containerized applications.
Configuration	Specified using –network=host.	Uses Docker’s default bridge network or custom bridges.

73. Why is Docker Considered the Best Container Platform?

Docker is widely regarded as the leading container platform due to:

• Ease of Use: Simplifies container creation, deployment, and management with intuitive commands.
• Portability: Ensures consistent environments across development, testing, and production.
• Vast Ecosystem: Offers a rich set of tools, integrations, and a large community.
• Efficiency: Lightweight containers reduce resource usage compared to virtual machines.
• Flexibility: Supports various orchestrators like Kubernetes and Docker Swarm.
• Extensive Registry: Docker Hub provides access to millions of pre-built images.

These strengths make Docker a preferred choice for modern application development and deployment.

74. How Can You Parse the Output of docker inspect to Deduce Conclusions?

Use docker inspect with JSON parsing tools like jq to extract specific information.

Example:

docker inspect my_container | jq ‘.[0].NetworkSettings.IPAddress’

Steps:

• Run docker inspect:

docker inspect [container_id]

• Parse with jq: Extract IP address, environment variables, volumes, etc.
• Example to get the container’s IP:

docker inspect my_container | jq ‘.[0].NetworkSettings.IPAddress’

75. What is the Best Way to Inspect a Docker Container?

The best way to inspect a Docker container is by using the docker inspect command.

Usage:

docker inspect [container_id]

Features:

• Provides detailed JSON output of container configurations.
• Includes information on networking, mounts, environment variables, and more.
• Can be combined with tools like jq for targeted data extraction.

Alternative: Use docker ps for a summary view of running containers.

This method offers comprehensive insights necessary for debugging and management.

76. What is the Difference Between a Docker Volume and a Bind Mount?

Aspect	Docker Volume	Bind Mount
Definition	Managed by Docker, stored in Docker’s directory.	Directly maps a host directory or file to the container.
Management	Easier to manage with Docker commands.	Requires manual management on the host.
Portability	More portable across different environments.	Less portable; tied to specific host paths.
Performance	Generally better performance on Docker-managed storage.	Performance depends on the host filesystem.
Use Cases	Persistent storage for databases, application data.	Development environments needing real-time code changes.

77. If You Run the Same Build Again Why Does It Take Very Little Time?

Docker caches each layer during the build process. If the same build is run again:

• Layer Caching: Docker reuses unchanged layers from the cache.
• Efficiency: Only modified steps are rebuilt, significantly reducing build time.
• Speed: Common steps are fetched from the cache without re-execution.

Example:

docker build -t myapp:latest .

If no changes are made to the Dockerfile or source files, the build uses cached layers, making it faster.

78. Can RUN Commands Inside a Dockerfile Be Interactive?

No, RUN commands in a Dockerfile cannot be interactive.

Explanation:

• Build-Time Execution: RUN commands execute during the image build process, which is non-interactive.
• Automation: Docker builds are automated and require no user input.
• Alternative: Use environment variables or scripts to handle configurations that would otherwise require interaction.

Example:

RUN echo “Setting up environment” && apt-get update

This ensures commands run seamlessly without manual intervention.

79. How Can You Override the ENTRYPOINT at Runtime?

Use the –entrypoint flag with the docker run command.

Syntax:

docker run –entrypoint [new_entrypoint] [image_name] [command]

Example: To override the ENTRYPOINT to bash:

docker run –entrypoint bash my_image

Explanation:

• –entrypoint: Replaces the default ENTRYPOINT specified in the Dockerfile.
• Flexibility: Allows running different commands or scripts without modifying the Docker image.

This method is useful for debugging or executing alternative processes within containers.

80. How Does Docker’s Overlay Network Differ from Bridge Network?

Aspect	Overlay Network	Bridge Network
Scope	Multi-host networking across different Docker hosts.	Single-host networking for containers on the same host.
Use Case	Suitable for Docker Swarm and multi-node setups.	Ideal for standalone Docker containers.
Communication	Enables containers on different hosts to communicate securely.	Allows containers on the same host to communicate.
Configuration	Requires Swarm mode or network plugins.	Automatically created by Docker on a single host.
Scalability	Highly scalable for distributed applications.	Limited to the resources of a single host.

81.  Compare Docker Compose and Docker Swarm.

Docker Compose and Docker Swarm serve different purposes in the Docker ecosystem.

Docker Compose:

• Purpose: Defines and manages multi-container Docker applications.
• Usage: Primarily used in development environments.
• Configuration: Uses docker-compose.yml files.
• Scaling: Limited to a single host.

Docker Swarm:

• Purpose: Provides native clustering and orchestration for Docker containers.
• Usage: Suitable for production environments.
• Configuration: Uses docker service commands and swarm-specific settings.
• Scaling: Supports multi-host scaling and high availability.

82.  How is a Docker Container Different from a Kubernetes Pod?

Docker Container and Kubernetes Pod are fundamental units in containerized environments but differ in scope and functionality.

Docker Container:

• Definition: A single running instance of a Docker image.
• Isolation: Encapsulates an application and its dependencies.
• Management: Managed individually using Docker commands.

Kubernetes Pod:

• Definition: The smallest deployable unit in Kubernetes, which can contain one or multiple containers.
• Shared Resources: Containers in a Pod share the same network namespace and storage volumes.
• Management: Managed collectively as part of Kubernetes’ orchestration.

83. How Does Docker Handle Port Mapping, and How is it Different from Port Forwarding?

Docker Port Mapping and Port Forwarding both involve directing traffic to specific ports, but they operate differently.

Docker Port Mapping:

• Function: Maps a container’s port to a port on the host machine.
• Usage: Specified using the -p or –publish flag.

Example:

docker run -p 8080:80 nginx

Maps host port 8080 to container port 80.

Port Forwarding:

• Function: Typically refers to configuring network devices or firewalls to direct traffic to specific ports on internal machines.
• Usage: Managed through router/firewall settings.
• Example: Forwarding external port 80 to an internal server’s port 8080.

84. Compare Docker Stack and Docker Compose.

Aspect	Docker Stack	Docker Compose
Purpose	Deploys multi-service applications to Docker Swarm clusters.	Defines and runs multi-container Docker applications on a single host.
Orchestration	Built for orchestration in Swarm mode.	Primarily for local development and single-host deployments.
Commands	Uses docker stack deploy, docker stack services.	Uses docker-compose up, docker-compose down.
Scalability	Supports scaling across multiple nodes.	Limited to scaling on a single Docker host.
Configuration	Uses docker-compose.yml but with swarm-specific extensions.	Uses docker-compose.yml for service definitions.
Use Case	Suitable for production deployments in clustered environments.	Ideal for development, testing, and small-scale deployments.

Conclusion

Preparing for a Docker interview requires a solid understanding of fundamental and advanced concepts. This comprehensive list of 75+ questions and answers will help you build confidence and effectively demonstrate your expertise. By mastering these topics, you’ll be well-equipped to tackle any Docker-related challenges during your interview.

Remember, hands-on experience is just as important as theoretical knowledge. Practice building, deploying, and managing Docker containers to reinforce what you’ve learned. Learn about developing applications and tools like Docker with the Certificate Program in Application Development powered by Hero Vired and gain professional certification. Good luck with your Docker journey and your upcoming interviews!