Custom OAuth token redirect URL is wrong

Description:

The custom OAuth config gives incorrect redirect url in the /token call. I'm using docker behind nginx, and the redirect url given is the the docker url directly, like: http://mydomain.com:3000/_oauth/myidp istead of correct https://mydomain.com:/_oauth/myidp.

The admin panel shows correct https URL though.

It works otherwise, I had to relax the checks in my IdP to get pass that URL issue. That is however bad security!

Steps to reproduce:

1. Configure a custom OAuth service in tha admin panel against some flexible IdP.
   (Sorry I cannot share what I'm using, its in-house IdP).
2. Do the auth until you get to /token phase. See the redirect URL in incorrect.

Expected behavior:

Redirect URL should match the one in Admin panel.

Actual behavior:

URL matches the proxy_pass config in NGINX.

Server Setup Information:

• Version of Rocket.Chat Server: 3.15.0 from docker:latest
• Operating System: Ubuntu 20.04
• Deployment Method: docker
• Number of Running Instances: 1

Client Setup Information

Firefox on Windows 10

Additional context

Is there any config flag to force the URL?

Relevant logs:

Start by tracing the custom OAuth configuration and /token flow, comparing the redirect URL shown in the admin panel with the URL produced behind the NGINX proxy_pass setup. Reproduce with a custom IdP or equivalent proxied HTTPS configuration; done means the token request uses the configured public HTTPS redirect URL rather than the Docker URL, with coverage for the proxied case.