title: How to pass container ip as ENV to other container in docker-compose file
body: <p>this is my docker-compose file: </p>
  
  <pre><code>version: '3.0'
  services:
   app-web:
    restart: always
    build: ./web
    environment:
      PG_HOST: $(APP_DB_IP)
      PG_PORT: 5432
    ports:
     - "8081:8080"
    links: 
     - app-db
  
   app-db:
    build: ./db
    expose: 
     - "5432"
  
    volumes:
     - /var/lib/postgresql/data
  </code></pre>
  
  <p>I want to pass to app-web the ip of app-db (Postgresql in this case) as ENV var so it can connect to the DB smoothly... any idea on how to achieve it?</p>