Add FORWARDED_FOR_HEADERS to the reverse-proxy config

[dgiebert]

Needed for reverse proxy configuration to show the real IP (especially needed for helm https://github.com/nextcloud/helm/pull/613)

Also defined here: https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/reverse_proxy_configuration.html#defining-trusted-proxies

[jessebot]

@dgiebert thank you for submitting this! I am not a maintainer on this repo, but wanted to gently ask if you could also include an update to one of these sections of the README:

• https://github.com/nextcloud/docker?tab=readme-ov-file#using-the-apache-image-behind-a-reverse-proxy-and-auto-configure-server-host-and-protocol
• https://github.com/nextcloud/docker?tab=readme-ov-file#auto-configuration-via-environment-variables

This just makes it easier for future users to also use that env var you're adding :)