Capstone part 1: containerize the app

The app we're shipping

A small notes API called notesy: Python, a server.py listening on port 8000, storing notes in Postgres, with a /health endpoint (lesson 9-2) that checks its database connection. The repo:

notesy/
├── server.py
├── requirements.txt
├── tests/
│   └── test_notes.py
└── .git/

The mission across two lessons, using only things you have learned:

1. Dockerfile → image (unit 3)
2. compose file with app + db, volume, healthcheck (units 4, 5, 9)
3. GitHub Actions pipeline: test → build → push → deploy (units 7, 8)

Step zero, before any building: .dockerignore, from lesson 3-3:

.git
__pycache__
.env

The Dockerfile

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
EXPOSE 8000
CMD ["python", "server.py"]

Every line is a lesson you've done: pinned slim base (3-1, 6-2), dependencies copied before code so edits don't reinstall packages (3-3 cache), EXPOSE as documentation, one CMD.

Build and smoke-test it locally:

docker build -t notesy:dev .
docker run --rm -p 8080:8000 notesy:dev

(--rm auto-removes the container when it stops, handy for throwaway runs.) It starts, then crashes: no database. Correct behavior, and exactly why compose exists.

The compose file

services:
  app:
    build: .
    ports:
      - "8080:8000"
    environment:
      DATABASE_URL: postgres://notesy:devpass@db:5432/notesy
    depends_on:
      db:
        condition: service_healthy
  db:
    image: postgres:16
    environment:
      POSTGRES_USER: notesy
      POSTGRES_PASSWORD: devpass
      POSTGRES_DB: notesy
    volumes:
      - dbdata:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD", "pg_isready", "-U", "notesy"]
      interval: 5s
      retries: 5
volumes:
  dbdata:

Read it as a tour of the course: service names as hostnames (4-2, 5-1), env config matching on both sides (5-2), a named volume so notes survive (4-1), and a real healthcheck gating startup with service_healthy (9-2). pg_isready is Postgres's own readiness probe command.

docker compose up -d, then curl localhost:8080/health answers ok. The stack is alive.