50 most commonly asked Docker interview questions

Basic Docker Concepts

1. What is Docker and why is it used?
2. What is a Docker container?
3. What is the difference between a Docker image and a container?
4. How is Docker different from a Virtual Machine (VM)?
5. What is Docker Hub?
6. What are the components of Docker architecture?
7. What is Docker Engine?
8. Explain the Docker container lifecycle.
9. How do you install Docker on your local machine? (Linux/Windows/Mac)
10. What are Docker namespaces and what role do they play?

Docker Commands & Operations

11. How do you list all running Docker containers?
12. How do you stop/remove a container?
13. How do you remove unused Docker images and containers?
14. How do you check logs of a Docker container?
15. How do you execute a command inside a running container?
16. What is the difference between docker run and docker start?
17. How do you tag and push an image to Docker Hub?
18. How do you inspect a Docker image or container?
19. What are bind mounts and how do you use them?
20. How do you clean up dangling images and containers?

Dockerfile & Building Images

21. What is a Dockerfile and why is it used?
22. Explain the difference between COPY and ADD in Dockerfile.
23. What is the purpose of CMD vs ENTRYPOINT?
24. How do you optimize Docker image size?
25. What are multi-stage builds and when would you use them?
26. How do you run health checks in Docker?
27. How can you set environment variables in Docker?
28. What are build cache and layer caching in Docker builds?
29. How would you handle versioning of Docker images?
30. What is a .dockerignore file and why is it important?

Networking & Storage

31. What are Docker networks and how do they work?
32. What are the types of Docker networks? (bridge, host, overlay, etc.)
33. How do you create and connect a custom network?
34. What are Docker volumes and their use cases?
35. What is the difference between volumes and bind mounts?
36. How do you manage persistent data in Docker?
37. How do you inspect volume configuration?
38. What is a tmpfs mount in Docker?
39. Explain inter-container communication using Docker networks.
40. How do you expose and map ports in Docker?

Docker Compose & Orchestration

41. What is Docker Compose and why use it?
42. How do you scale services using Docker Compose?
43. How do environment variables work in Docker Compose?
44. What are healthchecks in Docker Compose?
45. What is Docker Swarm and its purpose?
46. How does Docker Swarm differ from Kubernetes?
47. How do you initialize a Docker Swarm?
48. What are services and replicas in Swarm?
49. How do you add nodes to a Swarm cluster?
50. How do you update a running container in production safely? (rolling updates / blue-green)

Tips to Prepare

✔ Understand practical Docker workflows and hands-on commands.
✔ Prepare real-world use cases you’ve worked on (CI/CD, microservices, multi-container apps).
✔ Practice writing and explaining Dockerfiles, Compose files, and network configs.