Use `$proxy_add_x_forwarded_for` for custom locations

[Ziron]

Use $proxy_add_x_forwarded_for when setting X-Forwarded-For in custom locations to match how it is set for default location.

The proxy settings included in the default location uses $proxy_add_x_forwarded_for instead of $remote_addr since 052cb8f, which handles when NPM is behind multiple reverse proxies.

This commit makes sure the same setting is used when creating custom location blocks for a proxy host.

[nginxproxymanagerci[bot]]

Docker Image for build 4 is available on DockerHub:

nginxproxymanager/nginx-proxy-manager-dev:pr-4348

[!NOTE] Ensure you backup your NPM instance before testing this image! Especially if there are database changes. This is a different docker image namespace than the official image.

[!WARNING] Changes and additions to DNS Providers require verification by at least 2 members of the community!