Docker for Beginners: What, Why, and How

┌─────────────────────┐  ┌─────────────────────┐
│   App A   │  App B  │  │  App A  │  App B    │
├───────────┴─────────┤  ├─────────┴───────────┤
│  Guest OS │ Guest OS│  │   Docker Engine     │
├─────────────────────┤  ├─────────────────────┤
│    Hypervisor       │  │   Host OS           │
├─────────────────────┤  ├─────────────────────┤
│    Host OS          │  │    Hardware         │
├─────────────────────┤  └─────────────────────┘
│    Hardware         │
└─────────────────────┘
    Virtual Machines             Containers

# Ubuntu / Debian
sudo apt update
sudo apt install docker.io

# macOS / Windows
# Download Docker Desktop from docker.com

# Verify
docker --version
docker run hello-world

# Run an interactive Ubuntu container
docker run -it ubuntu bash

# Run Nginx in the background
docker run -d -p 8080:80 nginx

# Visit http://localhost:8080

FROM python:3.11-slim

WORKDIR /app

COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

COPY . .

EXPOSE 8000

CMD ["python", "app.py"]

docker build -t my-app .
docker run -p 8000:8000 my-app

# Images
docker images                         # list images
docker pull nginx                     # download image
docker rmi nginx                      # remove image

# Containers
docker ps                             # running containers
docker ps -a                          # all containers
docker run nginx                      # create + start
docker start CONTAINER_ID             # start existing
docker stop CONTAINER_ID              # stop
docker rm CONTAINER_ID                # remove

# Logs and exec
docker logs CONTAINER_ID              # view logs
docker logs -f CONTAINER_ID           # follow logs
docker exec -it CONTAINER_ID bash     # shell inside container

# Cleanup
docker system prune                   # remove unused containers, images, networks
docker system prune -a                # remove everything unused

# docker-compose.yml
services:
  web:
    build: .
    ports:
      - "8000:8000"
    depends_on:
      - db

  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: secret
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

docker compose up -d       # start all services
docker compose down        # stop and remove
docker compose logs -f     # follow all logs

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

FROM nginx:alpine
COPY --from=builder /app/dist /usr/share/nginx/html

# Layer caching optimization
COPY requirements.txt .      # COPY first, then RUN
RUN pip install -r requirements.txt  # cached if requirements.txt unchanged
COPY . .                     # only then copy source