docker-compose extension field syntax

docker-compose.yml

version: "3"

# See: https://hackernoon.com/a-better-way-to-develop-node-js-with-docker-cd29d3a0093
# This is the v3 syntax to achieve essentially the same thing. Note that if you launched this with a simple
# `docker-compose up` it would attempt to start every definition at the service level, which is not what you
# want! However, you can leverage the fact that `depends_on` launches the listed services prior to the
# dependent service, so in this example `docker-compose up myproject_web` would start the db container
# before the web container. Alternatively, you could just throw the stuff you don't want started with `up`
# into a separate YAML file, as the article demonstrates.
services:
  x-web-config:
    &web-config
    env_file:
      - .env
      - ./server/.env
    environment:
      NODE_ENV: development
    image: node:12
    volumes:
      - .:/app
    working_dir: /app

  install:
    << : *web-config
    command: ["yarn"]

  myproject_web:
    << : *web-config
    command: ["yarn", "start"]
    container_name: myproject_web
    depends_on:
      - myproject_db
    networks:
      - myproject
    ports:
      - 3000:3000
      - 3002:3002
    restart: unless-stopped
    stdin_open: true

  myproject_db:
    image: "postgres:11.5"
    restart: always
    container_name: myproject_db
    ports:
      - 5432:5432
    volumes:
      - ./data:/var/lib/postgresql/data
    environment:
      POSTGRES_DB: mydbname

networks:
  myproject:
    driver: bridge