I am learning Django by trying to create the polling app described on the Django's website. At the same time, I am using Docker for the first time and I have encountered problems trying to make PostgreSQL database, used by this app, persistent.

Following the tips on using Docker with Django, found on the Docker website, I am running two services with docker-compose: one handling the database and the other the app itself (at least that's how I understand it). I have tried mounting a volume at the /var/lib directory inside the database container, where I found PostgreSQL directory. Although this directory became persistent, which I checked by creating some dummy files, migrations done via the web service are being erased every time I restart those containers.

Below are my docker-copmpose.yml file and Django's settings.py. What should I do to make this database persistent?

#docker-compose.yml
version: '3'

services:
  db:
    image: postgres
    environment:
      - POSTGRES_DB=postgres
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=postgres
    volumes:
      - .:/code
      - .:/var/lib
  web:
    build: .
    command: python manage.py runserver 0.0.0.0:8000
    volumes:
      - .:/code
    ports:
      - "8000:8000"
    depends_on:
      - db

#(...)
# Database
# https://docs.djangoproject.com/en/2.2/ref/settings/#databases

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql',
        'NAME': 'postgres',
        'USER': 'postgres',
        'PASSWORD': 'postgres',
        'HOST': 'db',
        'PORT': 5432,
    }
}
#(...)