Here are comprehensive Docker interview questions organized by level:

---

Beginner Level

Concepts

Q1: What is Docker and why is it used?

Docker is an open-source containerization platform that packages applications and their dependencies into lightweight, portable containers — ensuring they run consistently across any environment (dev, staging, production).

Q2: What is the difference between a container and a virtual machine?

	Container	Virtual Machine
OS	Shares host OS kernel	Has its own OS
Size	Lightweight (MBs)	Heavy (GBs)
Startup	Seconds	Minutes
Isolation	Process-level	Full hardware-level
Performance	Near-native	Overhead

Q3: What is a Docker image vs a Docker container?

• Image — A read-only blueprint/template used to create containers
• Container — A running instance of an image

Q4: What is a Dockerfile?

A text file containing step-by-step instructions to build a Docker image automatically.

Q5: What is Docker Hub?

A public cloud-based registry where Docker images are stored, shared, and distributed.

---

Basic Commands

Q6: What are the most common Docker commands?

docker build -t myapp .                                  # Build image

docker run -d -p 8080:80 myapp                # Run container

docker ps                                                         # List running containers

docker ps -a                                                     # List all containers

docker stop <container_id>                           # Stop container

docker rm <container_id>                              # Remove container

docker images                                                 # List images

docker rmi <image_id>                                   # Remove image

docker logs <container_id>                           # View logs

docker exec -it <id> /bin/bash                     # Enter container shell

Q7: What is the difference between CMD and ENTRYPOINT?

	CMD	ENTRYPOINT
Purpose	Default command, easily overridden	Fixed command, always executes
Override	Yes, at runtime	Only with –entrypoint flag
Use case	Flexible defaults	Enforced commands

ENTRYPOINT [“python”]                     # always runs python

CMD [“app.py”]                                      # default arg, can be overridden

Q8: What is the difference between COPY and ADD?

• COPY — Simply copies files from host to container (preferred)
• ADD — Same as COPY but also supports URLs and auto-extracts tar files

---

Intermediate Level

Networking

Q9: What are Docker network types?

Network	Description	Use Case
bridge	Default, isolated network	Single host containers
host	Shares host network stack	High performance needs
none	No networking	Fully isolated containers
overlay	Multi-host networking	Docker Swarm / distributed apps

docker network create my-network

docker run –network my-network myapp

Q10: How do containers communicate with each other?

Containers on the same custom bridge network can communicate using their container name as hostname.

# Both containers on same network can reach each other by name

docker run –network my-net –name db postgres

docker run –network my-net –name app myapp  # app can reach “db”

---

Volumes & Storage

Q11: What is the difference between volumes, bind mounts, and tmpfs?

Type	Description	Use Case
Volume	Managed by Docker	Persistent data (databases)
Bind Mount	Maps host directory to container	Development, live code reload
tmpfs	Stored in memory only	Sensitive/temporary data

docker run -v myvolume:/data myapp          # volume

docker run -v /host/path:/container myapp   # bind mount

Q12: How do you persist data in Docker?

Use named volumes — data persists even after the container is removed.

docker volume create mydata

docker run -v mydata:/app/data myapp

---

Docker Compose

Q13: What is Docker Compose and when do you use it?

Docker Compose defines and runs multi-container applications using a single docker-compose.yml file.

version: “3.8”

services:

  app:

    build: .

    ports:

      – “8080:80”

    depends_on:

      – db

    environment:

      – DB_HOST=db

  db:

    image: postgres:15

    volumes:

      – pgdata:/var/lib/postgresql/data

    environment:

      – POSTGRES_PASSWORD=secret

volumes:

  pgdata:

docker-compose up -d      # Start all services

docker-compose down       # Stop and remove

docker-compose logs -f    # Follow logs

Q14: What is the difference between docker-compose up and docker-compose start?

• up — Creates and starts containers (builds if needed)
• start — Starts existing stopped containers only

---

Images & Optimization

Q15: How do you reduce Docker image size?

• Use minimal base images like alpine
• Use multi-stage builds
• Combine RUN commands to reduce layers
• Use .dockerignore to exclude unnecessary files

# Multi-stage build example

FROM node:18 AS builder

WORKDIR /app

COPY . .

RUN npm run build

FROM nginx:alpine

COPY –from=builder /app/dist /usr/share/nginx/html

Q16: What is a .dockerignore file?

Similar to .gitignore — tells Docker which files to exclude from the build context.

node_modules

.git

*.log

.env

dist

---

Advanced Level

Security

Q17: How do you secure Docker containers?

• Run containers as non-root user
• Use read-only filesystems where possible
• Scan images for vulnerabilities (docker scout)
• Limit container capabilities with –cap-drop
• Never store secrets in Dockerfiles — use Docker Secrets or environment variables

# Run as non-root

RUN adduser –disabled-password appuser

USER appuser

Q18: What is the difference between docker save and docker export?

	docker save	docker export
Target	Image	Container
Includes	All layers & history	Flattened filesystem only
Use case	Backup/transfer images	Snapshot a running container

---

Performance & Production

Q19: How do you limit container resources?

docker run \

  –memory=”512m” \

  –cpus=”1.0″ \

  –memory-swap=”1g” \

  Myapp

Q20: What is the difference between Docker Swarm and Kubernetes?

	Docker Swarm	Kubernetes
Complexity	Simple	Complex but powerful
Setup	Easy	Steeper learning curve
Scaling	Basic auto-scaling	Advanced auto-scaling
Community	Smaller	Very large
Best for	Small–medium workloads	Large enterprise workloads

Q21: What happens when a Docker container crashes?

Use restart policies to handle crashes automatically:

docker run –restart=always myapp         # Always restart

docker run –restart=on-failure:3 myapp   # Restart up to 3 times on failure

docker run –restart=unless-stopped myapp # Restart unless manually stopped

---

Dockerfile Best Practices

Q22: What are Dockerfile best practices?

# ✅ Good Dockerfile

FROM node:18-alpine                    # Use minimal base image

WORKDIR /app

COPY package*.json ./                  # Copy dependency files first

RUN npm ci –only=production           # Install dependencies

COPY . .                               # Copy source code

RUN adduser –disabled-password app    # Create non-root user

USER app                               # Switch to non-root

EXPOSE 3000

CMD [“node”, “server.js”]

---

Quick-Fire Questions

Question	Answer
Default Docker network?	bridge
Docker config file location?	/etc/docker/daemon.json
How to see container resource usage?	docker stats
How to copy files into a container?	docker cp file.txt container:/path
Difference between stop and kill?	stop = graceful (SIGTERM), kill = forceful (SIGKILL)
What is a dangling image?	An image with no tag, created by rebuilds
How to clean up unused resources?	docker system prune

---