Connection refused while connecting to upstream when using nginx as reverse proxy

The setup is as follows: I have a Gunicorn/Django app running on 0.0.0.0:8000 that is accessible via the browser. To serve static files I am running nginx as a reverse proxy. /etc/nginx/nginx.conf is configured to forward requests as follows:

server {
    location /static/ {
        alias /data/www/;
    }


    # Proxying the connections
    location / {
        proxy_set_header   Host $host;
        proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;

        proxy_redirect     off;
        proxy_pass         http://0.0.0.0:8000;
    }
}

and my docker-compose.yml file is as follows:

version: '3.3'

services:
  web:
    restart: always
    build: ./web
    expose:
      - "8000"
    ports:
      - "8000:8000"
    volumes:
      - staticdata:/usr/src/app/static_files
    command: gunicorn wsgi:application --workers 2 --bind 0.0.0.0:8000
    depends_on:
      - postgres

  nginx:
    restart: always
    build: ./nginx
    ports:
      - "80:80"
      - "443:443"
    volumes:
      - staticdata:/data/www
    depends_on:
      - web

  postgres:
    image: postgres:9.2
    restart: always
    volumes:
      - pgdata:/var/lib/postgresql/data
    ports:
      - "5432:5432"

volumes:
  staticdata:
  pgdata:

When I visit 0.0.0.0:8000 via the browser the application works fine (albeit without serving static files), but when I visit 127.0.0.1:80 I get the following error:

nginx_1     | 2017/09/17 13:59:46 [error] 6#6: *5 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: , request: "GET / HTTP/1.1", upstream: "http://0.0.0.0:8000/", host: "127.0.0.1"

I know that this error indicates that the server running on 0.0.0.0:8000 is not accepting requests, but since I can visit it via the browser I am a bit confused.

Thank you in advance.