Docker losing Postgres server data when I shutdown the backend container

I've got the following docker-compose file and it serves up the application on port 80 fine.

version: '3'

services:
    backend:
        build: ./Django-Backend
        command: gunicorn testing.wsgi:application --bind 0.0.0.0:8000 --log-level debug
        expose:
            - "8000"
        volumes:
            - static:/code/backend/static
        env_file:
            - ./.env.prod

    nginx:
        build: ./nginx
        ports:
            - 80:80
        volumes:
            - static:/static
        depends_on:
            - backend

    db:
        image: postgres
        environment:
            - POSTGRES_DB=postgres
            - POSTGRES_USER=postgres
            - POSTGRES_PASSWORD=postgres
        volumes:
            - postgres_data:/var/lib/postgresql/data/

volumes:
    static:
    postgres_data:

Once in there I can log into the admin and add an extra user which gets saved to the database as I can reload the page and the user is still there. Once I stop the backend docker container however that user is gone. Given Postgres is operating in a different container and I'm not bringing it down I'm unsure how stopping the backend container and restarting it is causing the data not to be available.

Thanks in advance.

EDIT: I'm bringing up the docker container with the following command.

docker-compose -f docker-compose.prod.yml up -d

I'm bringing down the container by just using docker desktop and stopping the container.

I'm running DJANGO 3 for the backend and I've also tried adding a superuser in the terminal when the container is running:

# python manage.py createsuperuser 
Username (leave blank to use 'root'): mikey
Email address: 
Password: 
Password (again): 
This password is too common.
Bypass password validation and create user anyway? [y/N]: y
Superuser created successfully.

Which works and the user appears while the container is running. However, once again when I shut the container down via docker desktop and then restart it that user that was just created is gone.

FURTHER EDIT:

settings.py using dotenv "from dotenv import load_dotenv"

DATABASES = {
    "default": {
        "ENGINE": os.getenv("SQL_ENGINE"),
        "NAME": os.getenv("SQL_DATABASE"),
        "USER": os.getenv("SQL_USER"),
        "PASSWORD": os.getenv("SQL_PASSWORD"),
        "HOST": os.getenv("SQL_HOST"),
        "PORT": os.getenv("SQL_PORT"),
    }
}

with the .env.prod file having the following values:

DEBUG=0
DJANGO_ALLOWED_HOSTS=localhost 127.0.0.1 [::1]
SQL_ENGINE=django.db.backends.postgresql
SQL_DATABASE=postgres
SQL_USER=postgres
SQL_PASSWORD=postgres
SQL_HOST=db
SQL_PORT=5432

SOLUTION:

Read the comments to see the diagnosis by other legends but updated docker-compose file looks like this. Note the "depends_on" block.

version: '3'

services:
    backend:
        build: ./Django-Backend
        command: gunicorn testing.wsgi:application --bind 0.0.0.0:8000 --log-level debug
        expose:
            - "8000"
        volumes:
            - static:/code/backend/static
        env_file:
            - ./.env.prod
        depends_on:
            - db

    nginx:
        build: ./nginx
        ports:
            - 80:80
        volumes:
            - static:/static
        depends_on:
            - backend

    db:
        image: postgres
        environment:
            - POSTGRES_DB=postgres
            - POSTGRES_USER=postgres
            - POSTGRES_PASSWORD=postgres
        volumes:
            - postgres_data:/var/lib/postgresql/data/
        expose:
            - "5432"

volumes:
    static:
    postgres_data:

FINAL EDIT:

Added the following code to my entrypoint.sh file to ensure Postgres is ready to accept connections by the backend container.

if [ "$DATABASE" = "postgres" ]
then
    echo "Waiting for postgres..."

    while ! nc -z $SQL_HOST $SQL_PORT; do
      sleep 0.1
    done

    echo "PostgreSQL started"
fi