Top 50 Docker Interview Question and Answers

1. What is Docker?
   Docker is a containerization platform that packages applications and dependencies into lightweight, portable containers that run consistently across various environments.

2. What is a Docker container?
   A container is a lightweight, portable, and self-contained package that includes everything needed to run an application, ensuring consistency across different environments.

3. How does Docker differ from virtual machines (VMs)?
   Unlike VMs, Docker containers share the host OS kernel, making them more lightweight and faster to start than VMs, which need a separate OS instance.

4. What is Docker Hub?
   Docker Hub is a public repository for Docker images, allowing users to share and access pre-built images.

5. What is a Dockerfile?
   A Dockerfile is a text file with instructions to build a Docker image. It defines the base image, dependencies, commands, and other configurations for the image.

6. Explain the concept of an image in Docker.
   A Docker image is a read-only template that contains the application and its dependencies, used to create containers.

7. How do you create a Docker image?
   Use the docker build command with a Dockerfile to create an image.

8. What is the difference between docker run and docker start?
   docker run creates and starts a new container, while docker start restarts an existing stopped container.

9. Explain the purpose of the docker-compose tool.
   Docker Compose is used to define and manage multi-container Docker applications with a single YAML configuration file (docker-compose.yml).

10. What is the purpose of a Docker namespace?
    Namespaces provide isolation to containers in terms of processes, network, and file systems.

11. How do you list all running Docker containers?
    Use docker ps to list running containers.

12. How do you stop a Docker container?
    Use docker stop <container_id> to stop a container.

13. How do you remove a Docker container?
    Use docker rm <container_id> to remove a container.

14. How do you remove a Docker image?
    Use docker rmi <image_id> to remove an image.

15. Explain docker pull and docker push.
    docker pull downloads an image from Docker Hub, while docker push uploads an image to a repository.

16. How do you check the logs of a Docker container?
    Use docker logs <container_id> to view container logs.

17. How do you execute a command inside a running container?
    Use docker exec -it <container_id> <command> to run a command inside a container.

18. What is the docker inspect command used for?
    docker inspect provides detailed information about a container or image, such as configuration and state.

19. How do you restart a Docker container?
    Use docker restart <container_id> to restart a container.

20. How do you tag a Docker image?
    Use docker tag <source_image> <target_image> to add a tag to an image.

21. What is Docker networking?
    Docker networking allows communication between containers within a host and across multiple hosts.

22. What are the different Docker network types?
    Bridge, host, overlay, and macvlan.

23. What is the default network in Docker?
    Bridge network is the default network for Docker containers.

24. How do you connect a container to a specific network?
    Use docker network connect <network_name> <container_id> to connect a container to a network.

25. What is Docker Compose networking?
    Docker Compose automatically creates a network for all services in the docker-compose.yml file, allowing containers to communicate.

26. How do you expose ports in Docker?
    Use the -p or --publish option in docker run to map container ports to the host.

27. What is the purpose of docker network create?
    docker network create is used to create a custom network for Docker containers.

28. How do containers in a bridge network communicate?
    Containers communicate through the bridge network’s IP addressing, isolating traffic from other networks.

29. Explain the overlay network in Docker Swarm.
    Overlay networks enable communication between Docker services across multiple hosts in a swarm.

30. What is a macvlan network?
    Macvlan allows containers to appear as physical devices on the network, providing unique MAC addresses for each container.

31. What is a Docker volume?
    A Docker volume is a persistent storage mechanism that allows data to be shared and retained independently of container lifecycles.

32. How do you create a Docker volume?
    Use docker volume create <volume_name> to create a volume.

33. How do you mount a volume in Docker?
    Use -v <volume_name>:<container_path> when running a container.

34. Explain bind mounts in Docker.
    Bind mounts map directories from the host system directly into the container, providing a way to share data between them.

35. How do you remove a Docker volume?
    Use docker volume rm <volume_name> to remove a volume.

36. What is the difference between volumes and bind mounts?
    Volumes are managed by Docker and stored separately, while bind mounts reference specific host directories.

37. Can you use multiple volumes in a single container?
    Yes, multiple volumes can be attached to a container using the -v option.

38. How does Docker handle data persistence?
    Data can persist in Docker through volumes or by using external storage solutions.

39. What is the purpose of docker volume inspect?
    It provides details about a specific volume, such as its mount point and configuration.

40. Explain the concept of tmpfs mount in Docker.
    A tmpfs mount is a temporary file system in memory, useful for storing non-persistent data in containers.

41. What is Docker Swarm?
    Docker Swarm is Docker’s native clustering and orchestration tool, used for managing containerized applications across a cluster.

42. How do you initialize a Docker Swarm?
    Use docker swarm init on the manager node to initialize a swarm.

43. What are Swarm services?
    Services are tasks that run containers across swarm nodes, managed by Docker Swarm.

44. What is the difference between Swarm and Kubernetes?
    Swarm is Docker’s native orchestration, simpler but less feature-rich, while Kubernetes is an advanced orchestration tool with complex features and broader adoption.

45. How do you add a node to a Docker Swarm?
    Use docker swarm join with the token provided by the manager node.

46. What is the purpose of docker service commands in Swarm?
    docker service commands create, update, and manage services in a swarm.

47. How does scaling work in Docker Swarm?
    Scaling increases or decreases the number of service replicas across the cluster.

48. What are Docker Swarm nodes?
    Nodes are machines in the swarm cluster; they can be managers or workers.

49. How does Docker handle container health checks?
    Docker checks the health status defined in the Dockerfile or docker-compose.yml to restart or manage unhealthy containers.

50. What are labels in Docker Swarm?
    Labels are metadata used to organize and manage resources in Swarm, like services and containers.