Docker & Containers – 50 Interview Questions & Answers

 

🔰 Basics of Containers (1–10)

1. What is a container?

Answer:
A container is a lightweight, isolated environment that packages an application with its dependencies and runs consistently across systems.

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2. What is Docker?

Answer:
Docker is a containerization platform used to build, ship, and run containers efficiently.

Docker container architecture diagram, image to container flow

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3. Difference between VM and Container?

Answer:

Virtual Machine	Container
Heavy	Lightweight
Own OS	Shares host OS
Slow startup	Fast startup

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4. Why are containers faster than VMs?

Answer:
Containers reuse the host OS kernel, avoiding full OS boot.

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5. What problems does Docker solve?

Answer:

• Environment mismatch

• Dependency conflicts

• Deployment inconsistency

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6. What is Docker Image?

Answer:
A read-only blueprint used to create Docker containers.

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7. What is Docker Container?

Answer:
A running instance of a Docker image.

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8. What is Docker Engine?

Answer:
The core service that builds and runs containers.

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9. What is Docker Hub?

Answer:
A public container registry for Docker images.

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10. What is container immutability?

Answer:
Containers are not modified after creation—new versions require new images.

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⚙️ Docker Core Concepts (11–25)

11. What is Dockerfile?

Answer:
A text file containing instructions to build a Docker image.

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12. Common Dockerfile instructions?

Answer:

• FROM

• RUN

• COPY

• WORKDIR

• CMD

• EXPOSE

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13. What is FROM used for?

Answer:
Defines the base image.

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14. What is CMD vs ENTRYPOINT?

Answer:

• CMD: default command

• ENTRYPOINT: fixed executable

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15. What is Docker layer?

Answer:
Each instruction creates a cached layer to speed up builds.

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16. What is .dockerignore?

Answer:
Prevents unnecessary files from being copied into image.

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17. What is Docker volume?

Answer:
Persistent storage outside container lifecycle.

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18. Difference between volume and bind mount?

Answer:
Volumes are managed by Docker; bind mounts map host paths.

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19. What is Docker network?

Answer:
Allows containers to communicate with each other.

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20. Types of Docker networks?

Answer:

• Bridge

• Host

• None

• Overlay

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21. What is container port mapping?

Answer:
Maps container ports to host ports.

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22. What is Docker registry?

Answer:
A storage system for Docker images.

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23. What is image tagging?

Answer:
Assigning version names to images (app:v1).

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24. What is Docker cache?

Answer:
Reuse of image layers to speed up builds.

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25. What is multi-stage build?

Answer:
Reduces image size by separating build and runtime stages.

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🧠 Advanced Docker (26–40)

26. What is Docker Compose?

Answer:
Tool to define and run multi-container applications.

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27. What is docker-compose.yml?

Answer:
YAML file defining services, networks, volumes.

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28. What is container orchestration?

Answer:
Automated container management (scaling, healing, deployment).

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29. Why Kubernetes over Docker alone?

Answer:
Docker runs containers; Kubernetes manages containers at scale.

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30. What is stateless vs stateful container?

Answer:

• Stateless: no stored data

• Stateful: uses volumes/databases

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31. What is container health check?

Answer:
Ensures container is running correctly.

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32. What is Docker security best practice?

Answer:

• Non-root user

• Minimal base images

• Secrets management

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33. What is Docker secret?

Answer:
Secure way to store passwords & API keys.

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34. What is image vulnerability scanning?

Answer:
Detecting security issues in images.

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35. What is Docker build context?

Answer:
Files sent to Docker daemon during build.

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36. What is container logging?

Answer:
Standard output captured by Docker logging drivers.

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37. What is Docker swarm?

Answer:
Docker’s native clustering & orchestration tool.

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38. What causes large Docker images?

Answer:

• Unused dependencies

• No multi-stage build

• Large base images

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39. How to reduce image size?

Answer:

• Alpine images

• Multi-stage builds

• Clean cache

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40. What is OCI?

Answer:
Open Container Initiative – defines container standards.

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🧪 Real-World & DevOps Scenarios (41–50)

41. How Docker helps CI/CD?

Answer:
Provides consistent build & deployment environments.

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42. How do you version Docker images?

Answer:
Semantic versioning (v1.0.0).

DevOps engineer using Docker, containers running on cloud servers

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43. How do you rollback a Docker deployment?

Answer:
Deploy previous image tag.

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44. How do containers communicate?

Answer:
Via Docker networks using service names.

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45. How do you debug a container?

Answer:
Using docker exec -it.

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46. How do you monitor containers?

Answer:
Prometheus, Grafana, Docker stats.

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47. How do you handle secrets in production?

Answer:
Env vars, Vault, Kubernetes Secrets.

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48. What happens if container crashes?

Answer:
Restart policies or orchestrator handles it.

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49. Docker vs Podman?

Answer:
Podman is daemon-less & rootless.

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50. Why Docker is critical for modern apps?

Answer:
Because of scalability, portability, and speed.

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🧩 100% WORKING DOCKER SCRIPTS

✅ Script 1: Simple Node.js App

app.js

const http = require('http');

http.createServer((req, res) => {
  res.end('Docker is working perfectly!');
}).listen(3000);

Dockerfile

FROM node:18-alpine
WORKDIR /app
COPY app.js .
EXPOSE 3000
CMD ["node", "app.js"]

Run

docker build -t node-test .
docker run -p 3000:3000 node-test

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✅ Script 2: Nginx Container

docker run -d -p 8080:80 nginx

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✅ Script 3: Docker Compose (Node + Redis)

docker-compose.yml

version: "3.9"
services:
  app:
    image: node:18
    command: node -e "console.log('App Running')"
  redis:
    image: redis

Run

docker-compose up