Docker & Containerization

Difficulty: Intermediate

Question

What is Docker? Explain images, containers, and how you'd containerize a Node.js application.

Answer

Docker packages applications with their dependencies into containers - lightweight, portable, isolated environments.

Key concepts: - Image: Read-only template (like a class). Built from Dockerfile. - Container: Running instance of an image (like an object). Isolated process. - Dockerfile: Instructions to build an image - Volume: Persistent storage (containers are ephemeral) - Docker Compose: Run multi-container applications

Why Docker? - 'Works on my machine' → works everywhere - Consistent dev/staging/prod environments - Easy scaling and deployment - Microservices architecture enabler

Code examples

Dockerfile for Node.js App

# Multi-stage build for production

# Stage 1: Build
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# Stage 2: Production
FROM node:20-alpine
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
COPY package*.json ./

# Don't run as root
USER node

EXPOSE 3000
CMD ["node", "dist/index.js"]

# .dockerignore
# node_modules
# .git
# .env
# dist

Multi-stage builds reduce image size. Stage 1 has all build tools, Stage 2 only has production files. The .dockerignore prevents copying unnecessary files.

Docker Compose for Full Stack

# docker-compose.yml
version: '3.8'
services:
  app:
    build: .
    ports:
      - '3000:3000'
    environment:
      - DATABASE_URL=postgresql://user:pass@db:5432/mydb
      - REDIS_URL=redis://cache:6379
    depends_on:
      - db
      - cache
    volumes:
      - ./src:/app/src  # hot reload in dev

  db:
    image: postgres:16-alpine
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: mydb
    volumes:
      - pgdata:/var/lib/postgresql/data
    ports:
      - '5432:5432'

  cache:
    image: redis:7-alpine
    ports:
      - '6379:6379'

volumes:
  pgdata:  # named volume persists data

# Commands:
# docker compose up -d      # start all services
# docker compose logs -f    # stream logs
# docker compose down       # stop all
# docker compose down -v    # stop + remove volumes

Docker Compose defines multi-container applications. Each service runs in its own container but can communicate via the internal network.

Essential Docker Commands

# Build and run
docker build -t myapp:1.0 .
docker run -d -p 3000:3000 --name myapp myapp:1.0

# List and manage
docker ps                    # running containers
docker ps -a                 # all containers
docker images                # local images
docker logs myapp            # container logs
docker exec -it myapp sh     # shell into container

# Cleanup
docker stop myapp            # stop container
docker rm myapp              # remove container
docker rmi myapp:1.0         # remove image
docker system prune -a       # remove everything unused

# Debugging
docker inspect myapp         # detailed container info
docker stats                 # CPU/memory usage
docker logs --tail 50 myapp  # last 50 log lines

These are the most commonly used Docker commands. Know them by heart for interviews and daily work.

Key points

• Alpine images are much smaller (~50MB vs ~900MB for full images)
• Always use .dockerignore to exclude node_modules, .git, .env
• Volumes persist data beyond container lifecycle

Concepts covered

Docker, Container, Image, Dockerfile, Docker Compose, Volume