Docker — images, containers, Dockerfile, docker-compose

FROM python:3.12-slim          # base image (ek chhota Debian + Python)
WORKDIR /app                   # /app mein cd (agar na ho toh create hota hai)
COPY requirements.txt .        # sirf manifest copy karo (cache-friendly)
RUN pip install -r requirements.txt   # deps install karo -> cached layer
COPY . .                       # ab source code copy karo
EXPOSE 8000                    # port document karta hai (publish NAHI karta)
CMD ["python", "app.py"]       # container start hone par default process

docker build -t myapp:1.0 .      # '.' mein Dockerfile se image build karo
docker run -d -p 8080:8000 myapp:1.0   # -d detached, host:container port map
docker ps                        # running containers list karo
docker exec -it <id> bash        # running container ke andar shell kholo
docker logs <id>                 # container ka stdout/stderr dekho
docker stop <id> && docker rm <id>     # pehle stop phir remove karo
docker images                    # images list karo
docker volume create data        # persistent storage

# docker-compose.yml
services:
  web:
    build: .                 # local Dockerfile se build karo
    ports:
      - "8080:8000"
    depends_on:
      - db                   # pehle db start karo
    environment:
      - DATABASE_URL=postgres://db:5432/mydb
  db:
    image: postgres:16
    volumes:
      - pgdata:/var/lib/postgresql/data   # restarts ke baad bhi DB persist karo
volumes:
  pgdata:                    # named volume container deletion ke baad bhi survive karta hai