1. What is Docker?
Docker is a platform to build, ship, and run applications in containers. It ensures consistency across environments.
2. What is a container?
A container is a lightweight, standalone package of software. It includes code, runtime, and dependencies.
3. What is a Docker image?
An image is a read-only template used to create containers. It contains application code and dependencies.
4. Difference between image and container?
Image is a blueprint; container is a running instance. Containers are created from images.
5. What is Dockerfile?
A Dockerfile is a script with instructions to build images. It automates the image creation process.
6. What is Docker Hub?
Docker Hub is a cloud-based repository for Docker images. You can store and share images here.
7. What is Docker Engine?
Docker Engine is the runtime that builds and runs containers. It includes daemon, API, and CLI.
8. What is Docker daemon?
It is a background service managing Docker objects. It handles containers, images, and networks.
9. What is Docker CLI?
CLI is the command-line interface to interact with Docker. It sends commands to the daemon.
10. What is virtualization?
Virtualization creates virtual machines on physical hardware. Each VM has its own OS.
11. Docker vs Virtual Machine?
Docker shares OS kernel; VMs run separate OS. Docker is faster and lightweight.
12. What is a Docker registry?
A registry stores Docker images. Example: Docker Hub.
13. What is a private registry?
It is a secured registry for internal use. Used by organizations.
14. What is docker pull?
It downloads an image from a registry. Used before running containers.
15. What is docker run?
It creates and starts a container. Combines create + start.
16. What is docker ps?
Lists running containers. Use -a to show all containers.
17. What is docker stop?
Stops a running container. Gracefully shuts it down.
18. What is docker rm?
Removes a container. Works only on stopped containers.
19. What is docker images?
Lists all local Docker images. Shows repository and size.
20. What is docker rmi?
Deletes an image from local storage. Frees up disk space.
21. What is a Docker volume?
Volumes store persistent data outside containers. Data remains even after container deletion.
22. What is bind mount?
It links a host directory to a container. Used for development.
23. What is Docker network?
It enables communication between containers. Supports multiple network types.
24. Types of Docker networks?
Bridge, Host, Overlay, None. Each serves different use cases.
25. What is bridge network?
Default network for containers. Allows internal communication.
26. What is host network?
Container shares host network. No isolation.
27. What is overlay network?
Used in Docker Swarm for multi-host communication. Connects containers across nodes.
28. What is Docker Compose?
Tool to define multi-container apps. Uses YAML configuration file.
29. What is docker-compose.yml?
A config file for services, networks, volumes. Used by Docker Compose.
30. What is docker build?
Builds an image from a Dockerfile. Creates custom images.
31. What is layer in Docker?
Each instruction creates a layer. Layers improve caching and efficiency.
32. What is caching in Docker?
Reuses unchanged layers during builds. Speeds up image creation.
33. What is .dockerignore?
Excludes files from build context. Reduces image size.
34. What is container lifecycle?
States: create, run, pause, stop, delete. Defines container behavior.
35. What is port mapping?
Maps container port to host port. Used to access apps externally.
36. What is -d flag?
Runs container in detached mode. Works in background.
37. What is -p flag?
Used for port mapping. Example: 8080:80.
38. What is environment variable in Docker?
Used to pass config into containers. Defined using -e.
39. What is Docker Swarm?
Docker’s native clustering tool. Manages multiple nodes.
40. What is Kubernetes?
Container orchestration platform. Automates deployment and scaling.
41. What is image tagging?
Assigning version labels to images. Example: app:v1.
42. What is latest tag?
Default tag if none specified. Represents latest version.
43. What is container isolation?
Containers run independently. Uses namespaces and cgroups.
44. What are namespaces?
Provide isolation in Linux. Separate processes and resources.
45. What are cgroups?
Control resource usage. Limits CPU, memory, etc.
46. What is multi-stage build?
Uses multiple stages in Dockerfile. Reduces final image size.
47. What is Docker inspect?
Shows detailed container info. Returns JSON output.
48. What is Docker logs?
Displays container logs. Used for debugging.
49. What is Docker exec?
Runs command inside container. Example: bash access.
50. Why use Docker?
Ensures consistency across environments. Simplifies deployment and scaling.
Conclusion
Docker has become a fundamental skill for modern developers and DevOps engineers, especially at the fresher level where understanding core concepts makes a strong first impression. By preparing these commonly asked interview questions, you not only build confidence but also gain practical clarity on how containerization works in real-world scenarios.
Focus on understanding concepts rather than memorizing commands, and try practicing with small projects to reinforce your knowledge. With consistent preparation, you’ll be well-equipped to handle Docker interview questions and take a solid step forward in your tech career.
