version: '2'

services:
db:
    image: eg_postgresql
    expose:
        - 5432
    environment:
        - POSTGRES_PASSWORD=docker
        - POSTGRES_USER=docker
        - POSTGRES_DB=postgres
web:
    build: .
    command: python3 manage.py runserver 0.0.0.0:8000
    volumes:
        - .:/test_application
    ports:
        - "8000:8000"
    links:
        - "db:db"
    environment:
        - DATABASE_URL=postgres://docker:docker@db:5432/postgres
        - DJANGO_SECRET_KEY=x7-g-xu^h5k%h8860!7ksn=@)7q9frn9_l6tmefvf)y=0)d!uh

conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
django.db.utils.OperationalError: could not connect to server: Connection refused
Is the server running on host "localhost" (127.0.0.1) and accepting
TCP/IP connections on port 5432?
could not connect to server: Cannot assign requested address
Is the server running on host "localhost" (::1) and accepting
TCP/IP connections on port 5432?

ubuntu@ip-172-31-7-117:~/dj1/helloworld$ sudo docker-compose ps
  Name                    Command               State           Ports

----------------------------------------------------------------------------------
helloworld_db_1    /usr/lib/postgresql/9.3/bi ...   Up      5432/tcp

helloworld_web_1   python3 manage.py runserve ...   Up      0.0.0.0:8000->8000/tcp