django with docker , TCP/IP connections on port 5432 error

I have a django application. And the application works fine. But now I try to dockkerize the django application. So my docker file looks like:

# pull official base image
FROM python:3.9-alpine3.13

# set work directory
WORKDIR /usr/src/app

# set environment variables
ENV PYTHONDONTWRITEBYTECODE 1
ENV PYTHONUNBUFFERED 1

# install psycopg2 dependencies
RUN apk update 
    && apk add linux-headers postgresql-dev gcc python3-dev musl-dev



# install dependencies

RUN pip install --upgrade pip
COPY ./requirements.txt .
COPY ./requirements.dev.txt . 
RUN pip install -r requirements.txt


# copy entrypoint.sh
COPY ./entrypoint.sh .
RUN sed -i 's/r$//g' /usr/src/app/entrypoint.sh
RUN chmod +x /usr/src/app/entrypoint.sh

# copy project
COPY . .

# run entrypoint.sh
ENTRYPOINT ["/usr/src/app/entrypoint.sh"]

And dockercompose looke like:

version: '3.9'

services:
  app:
   build:
     context: .
     args:
       - DEV=true
   ports:
      - "8000:8000"
   volumes:
      - .:/app
   command: >
      sh -c "python manage.py migrate &&      
             python app/manage.py runserver 192.168.1.135:8000"
   env_file:
      - variables.env

   depends_on:
      - db

  db:
    image: postgres:13-alpine
    container_name: postgres
    volumes:
      - dev-db-data:/var/lib/postgresql/data
    environment:
      - POSTGRES_DB=db
      - POSTGRES_USER=user
      - POSTGRES_PASSWORD=password
    ports:
      - '5432:5432'
      
volumes:
    dev-db-data:
    dev-static-data:

So when I do: docker-compose up -d –build

I see that the db container is running. But the app container is not running.

And this error occurs:

django.db.utils.OperationalError: could not connect to server: Connection refused
dwl_backend-app-1  |    Is the server running on host "localhost" (127.0.0.1) and accepting
dwl_backend-app-1  |    TCP/IP connections on port 5432?
dwl_backend-app-1  | could not connect to server: Address not available
dwl_backend-app-1  |    Is the server running on host "localhost" (::1) and accepting
dwl_backend-app-1  |    TCP/IP connections on port 5432?

And of course I googled. And I found something that I have to change the postgresql.conf file and set the port to 5432. But port number is setting correct.

Question: how to tackle this error?

This i have in settings.py file:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': environ.get("DATABASE_NAME"),
        'USER': environ.get("DATABASE_USER"),
        'PASSWORD': environ.get("DATABASE_PASSWORD"),
        'HOST': environ.get("DATABASE_HOST"),
        'PORT': environ.get("DATABASE_PORT"),
    }
}