docker compose: a stack in one file

Here: what compose.yaml describes and what compose creates from that file.

docker compose reads compose.yaml in the current directory. Each entry under services: is a service: compose creates a container from it. A service's keys mirror the options of docker run:

services:
  web:
    image: nginx:alpine
    ports:
      - "8080:80"        # -p 8080:80
    depends_on:
      - app              # start after app
  app:
    image: busybox
    command: ["sh", "/app/run.sh"]
    environment:
      LOG_LEVEL: info    # -e LOG_LEVEL=info
    volumes:
      - ./app:/app:ro    # a directory next to the file
    restart: unless-stopped

The project name is the directory name. The lesson's stack in ~/stack has the services proxy and backend, and their containers are called stack-proxy-1 and stack-backend-1. compose creates the network stack_default for the project and connects every service to it. A service name works there like a container name in a Docker network of your own: proxy finds backend by name. The networks: key gives a service other networks — services see each other only when they share one.

Stack commands are run in the directory with the file: docker compose ps shows the state of the services, docker compose logs <service> shows one service's log, and with --tail N its last lines.