I’ve sent a PR to address the get-url issue:
https://github.com/discourse/discourse/pull/12284
About the 404 on /f/faq and /f/favicon/proxied, when I CURL these routes inside the container, bypassing Nginx, it works. I was able to fix it by changing the location directive described in Subfolder support with Docker from location /subfolder to location /subfolder/.
The nav bar showing both Guidelines + FAQ is the expected behavior, it’s unlikely the user will override the faq url with the same url.