Docker has revolutionized software development by enabling lightweight, portable, and consistent application deployment through containerization. For software engineers, understanding Docker is critical not only for day-to-day tasks but also for excelling in technical interviews. This guide consolidates essential Docker concepts, commands, and best practices into a structured format tailored for interview preparation.

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1. Introduction to Docker

What is Docker?

Docker is an open-source platform that automates application deployment using containers—lightweight, isolated environments that package code, dependencies, and configurations. Containers share the host OS kernel, making them faster and more resource-efficient than virtual machines (VMs).

Why Docker?

• Portability: Run consistently across environments (dev, test, production).
• Efficiency: Reduced overhead compared to VMs.
• Scalability: Easily replicate containers for horizontal scaling.
• Isolation: Processes and resources are segregated between containers.

Key Components of Docker

1. Docker Engine: Core service managing containers, images, networks, and volumes.
2. Docker Images: Immutable templates (e.g., nginx:latest) used to create containers.
3. Docker Containers: Runtime instances of images.
4. Docker Hub: Public registry for sharing images.
5. Docker Volumes: Persistent storage for container data.

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2. Core Concepts

Docker Image vs. Container

Image	Container
Static template (read-only)	Runtime instance (read-write)
Built via docker build	Started via docker run
Example: ubuntu:20.04	Running Ubuntu instance

Dockerfile

A text file with instructions to automate image creation:

dockerfile

FROM alpine:latest  
RUN apk add --no-cache nginx  
COPY index.html /var/www/html/  
CMD ["nginx", "-g", "daemon off;"]  

Key Instructions:

• FROM: Base image.
• RUN: Execute commands during build.
• COPY: Add files from host to image.
• CMD: Default command to run.

Docker Hub

• Pull an image: docker pull nginx:latest
• Push an image: docker push username/repo:tag

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3. Managing Images and Containers

Essential Commands

• Build an image:bashCopydocker build -t my-app:1.0 .
• List images/containers:bashCopydocker images # List images docker ps -a # List all containers
• Start/stop containers:bashCopydocker run -d -p 8080:80 nginx # Create and start docker start <container-id> # Restart stopped container
• Delete resources:bashCopydocker rmi <image-id> # Remove image docker rm <container-id> # Remove container

Lifecycle of a Container

1. Create: docker create
2. Start: docker start
3. Stop: docker stop (graceful) or docker kill (forceful)
4. Remove: docker rm

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4. Docker Networking

Network Types

1. Bridge: Default network for inter-container communication.
2. Host: Shares host’s network stack (no isolation).
3. Overlay: Connects containers across multiple hosts (e.g., Swarm).
4. Macvlan: Assigns MAC addresses to containers.

Example: Create a custom network and connect containers:

bash

docker network create my-net  
docker run --network my-net --name app1 nginx  

Port Mapping

Expose container ports to the host:

bash

docker run -p 8080:80 nginx   # Host:8080 → Container:80  

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5. Storage with Volumes

Volumes vs. Bind Mounts

Volume	Bind Mount
Managed by Docker	Maps host directory
Persistent and shareable	Tied to host filesystem

Example: Create and mount a volume:

bash

docker volume create my-vol  
docker run -v my-vol:/data alpine  

Managing Volumes

• List volumes: docker volume ls
• Remove unused volumes: docker volume prune

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6. Docker Compose

Define Multi-Container Apps

Example docker-compose.yml:

yaml

version: '3.8'  
services:  
  web:  
    image: nginx  
    ports:  
      - "8080:80"  
  db:  
    image: postgres  
    environment:  
      POSTGRES_PASSWORD: password  

Key Commands:

• Start services: docker-compose up
• Scale services: docker-compose up --scale web=3
• Override configs: Use docker-compose.override.yml.

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7. Security Best Practices

1. Isolation: Use namespaces and cgroups.
2. Non-Root Users:dockerfileCopyRUN useradd -m appuser USER appuser
3. Docker Content Trust (DCT): Verify image integrity with:bashCopyexport DOCKER_CONTENT_TRUST=1
4. Secrets Management: Store sensitive data using docker secret.

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8. Advanced Topics

Docker Swarm vs. Kubernetes

Docker Swarm	Kubernetes
Simpler setup	Complex but highly scalable
Built-in service discovery	Extensive ecosystem

Deploy in Swarm:

bash

docker swarm init  
docker service create --name web -p 8080:80 nginx  

Multi-Stage Builds

Optimize image size by separating build and runtime stages:

dockerfile

# Build stage  
FROM golang:1.17 AS builder  
WORKDIR /app  
COPY . .  
RUN go build -o app  

# Runtime stage  
FROM alpine:latest  
COPY --from=builder /app/app .  
CMD ["./app"]  

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9. Troubleshooting

Common Issues & Fixes

1. Container Crashes:
   • Check logs: docker logs <container-id>
   • Debug interactively: docker exec -it <container-id> /bin/bash
2. Network Issues:
   • Inspect network: docker network inspect my-net
   • Test connectivity: docker exec app1 ping app2
3. Storage Errors:
   • Resolve “No space left”:bashCopydocker system prune # Clean unused resources

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10. Key Takeaways for Interviews

1. Core Concepts: Master image-container differences, Dockerfile, and lifecycle.
2. Commands: Know docker run, docker-compose, and troubleshooting tools like docker logs.
3. Security: Highlight non-root users, DCT, and secrets.
4. Advanced Skills: Discuss Swarm, multi-stage builds, and multi-architecture images.

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By internalizing these concepts and practicing commands, you’ll confidently tackle Docker interview questions. Focus on real-world scenarios, optimize your Dockerfiles, and stay updated with industry trends. Happy containerizing! 🐳