Django-environ and Postgres environment for docker

I am using django-environ package for my Django project. I provided the DB url in the .env file, which looks like this: DATABASE_URL=psql://dbuser:dbpassword@dbhost:dbport/dbname

My DB settings in settings.py:

DATABASES = {
    "default": env.db(),
}

So far, I have no issues.

Then, I created a docker-compose.yml where I specified that my project uses Postgres database, i.e.:

version: '3.8'

services:
  ...
    db
      image: postgres:13
    volumes:
      - postgres_data:/var/lib/postgresql/data/
    environment:
      - POSTGRES_USER=???
      - POSTGRES_PASSWORD=???
      - POSTGRES_DB=???
      - "POSTGRES_HOST_AUTH_METHOD=trust"

Now I am confused a little.

How do I provide these POSTGRES_* env. variables there? Do I need to provide them as separate variables alongside with the DATABASE_URL in my .env file? If yes, what's the best way do accomplish this? I aim to avoid duplication in my settings.

You can use variable expansion in your .env file. Something like

DB_NAME=dbname
DB_USER=dbuser
DB_PASSWORD=dbpassword
DATABASE_URL=psql://$DB_USER:$DB_PASSWORD@dbhost:dbport/$DB_NAME

and then something like this in your compose file

services:
  postgresdb:
    container_name: projectname_db
    image: postgres:15
    environment:
      POSTGRES_DB: ${DB_NAME}
      POSTGRES_USER: ${DB_USER}
      POSTGRES_PASSWORD: ${DB_PASSWORD}
    ports:
      - "127.0.0.1:5432:5432"
...

I am not exactly familiar with django-environ but this should work

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