This chapter's application is a workshop signup service: a Flask app in ~/signup that stores names in Postgres.

~/signup
├── app.py            Flask app, listens on $PORT (5000), connects to $DB_HOST (db)
├── requirements.txt  flask, psycopg
├── Dockerfile        FROM python:3.12-slim, pip install, CMD python app.py
└── compose.yaml

image or build

A service needs an image. It either names one or builds one.

services:
  db:
    image: postgres:16          # pulled from the registry if missing

  web:
    build: .                    # docker build with ./ as the context

build: . is short for a mapping. Use the long form when the Dockerfile has another name or the context is elsewhere:

    build:
      context: ./services/web
      dockerfile: Dockerfile.prod

docker compose up builds a missing image once. After that it reuses it, even if app.py changed. Lesson 8 covers rebuilding.

ports

ports is a list of "HOST:CONTAINER" strings, the same format as docker run -p:

    ports:
      - "8000:5000"              # host 8000 → container 5000
      - "127.0.0.1:9000:9000"    # only reachable from the host itself

Services reach each other on the project network by service name and container port (db:5432). Publish a port only when something outside Docker needs it. Here db publishes nothing.

environment and env_file

environment sets variables in the container, like -e. It takes a mapping or a list:

    environment:
      DB_PASSWORD: workshop-pass
      PORT: "5000"

    environment:
      - DB_PASSWORD=workshop-pass
      - PORT=5000

env_file loads variables from a file, like --env-file. One KEY=value per line; # starts a comment.

    env_file:
      - web.env

When a variable is set in several places, the container gets one value:

Check what reached the container:

docker compose exec web env

The web service

services:
  web:
    build: .
    ports:
      - "8000:5000"
    environment:
      DB_PASSWORD: workshop-pass
    env_file:
      - web.env