Django + Docker: connection to server at "localhost" (127.0.0.1), port 5432 failed

I am trying to run my Django app (Nginx, Gunicorn) in docker.

But for request http://167.99.137.32/admin/ I have error: (full log https://pastebin.com/0f8CqCQM)

onnection to server at "localhost" (127.0.0.1), port 5432 failed: Connection refused
    Is the server running on that host and accepting TCP/IP connections?
connection to server at "localhost" (::1), port 5432 failed: Address not available
    Is the server running on that host and accepting TCP/IP connections?

I was trying answers from Can't run the server on Django (connection refused) but didn't solve my problem

settings.py

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'lk_potok_2',
        'USER': 'postgres',
        'PASSWORD': 'post222',
        'HOST': 'localhost',
        'PORT': 5432,
    },

docker-compose.yml

version: '3.9'

services:
  django:
    build: . # path to Dockerfile
    command: sh -c "gunicorn --bind 0.0.0.0:8000 potok.wsgi:application"
    volumes:
      - .:/project
      - static:/project/static
    expose:
      - 8000
    environment:
      - DATABASE_URL=postgres://postgres:post222@localhost:5432/lk_potok_2"
      - DEBUG=1

  db:
    image: postgres:13-alpine
    volumes:
      - pg_data:/var/lib/postgresql/data/
    expose:
      - 5432
    environment:
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=post222
      - POSTGRES_DB=lk_potok_2

  nginx:
    image: nginx:1.19.8-alpine
    depends_on:
      - django
    ports:
      - "80:80"
    volumes:
      - static:/var/www/html/static
      - ./nginx-conf.d/:/etc/nginx/conf.d

volumes:
    pg_data:
    static:

nginx-conf.nginx

upstream app {
    server django:8000;
}

server {
    listen 80;
    server_name 167.99.137.32;

    location / {
        proxy_pass http://django:8000;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $host;
        proxy_redirect off;
    }

    location /static/ {
        alias /var/www/html/static/;
    }
}

I was trying sudo systemctl start postgresql and sudo systemctl enable postgresql (the same error)

The postgres database is no longer running at localhost. In your case (since you named the container db) it is db.

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'lk_potok_2',
        'USER': 'postgres',
        'PASSWORD': 'post222',
        'HOST': 'db',
        'PORT': 5432,
    },

I don't really see why you would add this in here:

environment:
      - DATABASE_URL=postgres://postgres:post222@localhost:5432/lk_potok_2"

since you don't use it in your settings.py. But here it wil also have to be db instead of localhost.

--EDIT--

Explanation as why docker can recognise the other containers can be found here.