docker-compose postgresql implementation in python app

Question

I am building a Python app and I am trying to run it inside Docker. (My works completely in virtualenv with no Docker.)

So, my app consists of config.py file and other files which make everything work, but are in this case unimportant.

My config file looks like this:

SQLALCHEMY_DATABASE_URI = "postgresql+psycopg2://gimtest:gimtest@localhost:5432/gimtest-users"

App requires postgresql to be run before the app starts, so I tried using Docker-Compose. My docker-compose.yml looks like this:

version: '2'
services:
   web:
      build: ./web
      ports:
         - "5000:5000"
      depends_on:
         - postgres
      links:
         - postgres
         - elastic
      expose:
         - 5000
      command:
         "python run.py"
   postgres:
      image: postgres:9.5
      expose:
         - 5432
      environment:
         POSTGRES_USER: "gimtest"
         POSTGRES_PASSWORD: "gimtest"
         POSTGRES_DB: "gimtest-users"

   elastic:
      image: elasticsearch:2.3

Now the problem is that I can't connect to postgresql database. (At least that's what compile says because it throws an error from python app that no postgresql is running on specified url (up in config.py).

How can I fix the url so that it will work?