Forward to Django the public port where Nginx is exposed

[giohappy]

Original PR #32 from @nicokant

Nginx configuration uses $host variable that doesn't provide custom ports, causing errors with request.build_absolute_url. This PR aims to fix providing also non-standard port specified as HTTP_PORT/HTTPS_PORT.

Related issues:

• Custom port is ignored on Docker geonode-project#386
• Changing SITEURL to localhost:Port didn't change Django Rest Framework URLs geonode-project#217
• Changing HTTP_PORT to a value different from 80 is not working geonode-project#290
• standing up docker compose stack on a port different than 80 geonode#11734