Top 30 Docker Interview Questions and Answers for All Experience Levels

The command docker ps -a lists the status of all containers, both running and stopped.[3][5]

5. What command shows only running Docker containers?

Use docker ps to view all currently running containers.[3][4]

6. How do you start a stopped Docker container?

Run docker start <container_id> to start a stopped container by its ID or name.[4]

7. What is the difference between docker stop and docker kill?

docker stop sends a SIGTERM signal allowing graceful shutdown. docker kill sends SIGKILL for immediate termination.[4][2]

8. How do you run a Docker container in detached mode?

Use docker run -d <image_name>. The -d flag runs the container in detached (background) mode.[4]

9. What does the docker run command do?

docker run creates and starts a container from a specified image. Use docker create to only create without starting.[2][3]

10. When do you lose data stored in a Docker container?

Data persists in a container until the container is explicitly deleted using docker rm. Stopping or restarting doesn't delete data.[3][5]

Intermediate Docker Interview Questions (11-20)

11. What is a Dockerfile?

A Dockerfile is a text script containing instructions to build a Docker image layer by layer automatically.[1]

12. Explain the difference between COPY and ADD in Dockerfile.

COPY copies files from source to destination. ADD does the same but also supports URL downloads and automatic tar extraction.[2]

13. What is the difference between CMD and ENTRYPOINT in Dockerfile?

CMD provides default arguments for the container's main process (can be overridden). ENTRYPOINT configures the main executable and cannot be easily overridden.[2]

14. What does EXPOSE do in a Dockerfile?

EXPOSE documents which ports the container listens on but doesn't publish them. Use -p flag during docker run to publish ports.[1][2]

15. What is the difference between "expose" and "publish" in Docker?

EXPOSE in Dockerfile documents ports. PUBLISH (-p flag) makes container ports accessible from the host.[2]

16. How do you build a Docker image from a Dockerfile?

Use docker build -t <image_name> .. The . specifies the build context (current directory containing Dockerfile).[1]

17. What command removes all stopped containers and unused Docker objects?

docker system prune removes stopped containers, unused networks, build caches, and dangling images.[3]

18. What are Docker volumes and why use them?

Docker volumes provide persistent storage independent of container lifecycle. Data in volumes survives container deletion.[1]

19. How do you access a running container's shell?

Use docker exec -it <container_id> /bin/bash. The -it flags enable interactive terminal access.[6]

20. What is a Docker registry and what's the difference from a repository?

Docker registry stores images (like Docker Hub). A repository is a collection of related images with different tags within a registry.[2][5]

Advanced Docker Interview Questions (21-30)

21. What is Docker Compose and when would you use it?

Docker Compose manages multi-container applications using YAML files. Use it for defining services, networks, and volumes for complex apps.[6]

22. How do you control service startup order in Docker Compose?

Use depends_on in docker-compose.yml to specify startup sequence between services.[2]

23. What is a Docker image build cache?

Build cache stores intermediate layers from previous builds to speed up subsequent image builds by reusing unchanged layers.[2]

24. How do you transfer a Docker image between machines without a registry?

Save image to tar file with docker save -o image.tar <image>, transfer file, then load with docker load -i image.tar

25. What are orphan volumes in Docker and how to remove them?