Add content for dockerfile

[MichaelCurrin]

## Fields

Setting fields like `dockerfile`, `command`, `volume` and `environment`.

```yaml
services:
  mya_app:
    build:
      context: ..
      dockerfile: .devcontainer/Dockerfile
    command: sleep infinity
    volumes:
      - ..:/workspace
    environment:
      - DEBUG=0
      - ENVIRONMENT=local
    ports:
      - "8000:8000"
      - "8081:8081"
    depends_on:
      - my_app_db
```

For external services, it can be shorter:

```yaml
services:
  my_app_db:
    image: postgres:14
    volumes:
      - my_app_db_data:/var/lib/postgresql/data
    ports:
      - "5432:5432"
    healthcheck:
      test: [ "CMD-SHELL", "pg_isready -U commodities" ]
      interval: 5s
      timeout: 5s
      retries: 5

  redis:
    image: redis:alpine
    ports:
      - "6379:6379"

  nginx:
    container_name: nginx
    image: nginx:1.18
    ports:
      - "80:80"
    volumes:
      - "./nginx.conf:/etc/nginx/conf.d/default.conf"
```

You can set credentials in `environment` for a DB but it could be better to use unversioned secrets file instead.