How can I force gitlab sign in page to use https instead of http (with no redirect)?

  docker, gitlab-ce

I run a gitlab-ce (13.9.2) docker container on a Synology NAS.
The container starts correctly: gitlab is accessible via http on ip NAS IP port 8080.

In order to be able to access gitlab web interface via https on url git.nashostname port 443, I have then configured a reverse proxy on the Synology and have opened the port 443 (and blocked port 80) in the firewall. I’ve also set external_url = 'https://git.nashostname' in gitlab.rb configuration file then did a gitlab-ctl reconfigure.

Note: I do it this way in order to benefit of the certificates management feature of the Synology (no need to place the certificate and key on a shared folder).

When I restart the container, if I try to connect to https://git.nashostname, everything seems to be working fine during the startup of the container (i.e. I see the page with gitlab logo and the error 502 page), but then when the container is fully started, https://git.nashostname redirects to http://git.nashostname/users/sign_in (http) and then I’m stuck.

If I enter https://git.nashostname/users/sign_in manually (https instead of http) everything works fine.

The question is the following: how can I force the redirect from https://git.nashostname to https://git.nashostname/users/sign_in instead of http://git.nashostname/users/sign_in?

Note: I don’t want to allow access to http port 80, then redirect to https port 443.

Thank you a lot for your help

Source: Docker Questions