• I've read the guidelines for Contributing to YOURLS Projects
• This request isn't a duplicate of an existing issue
• I've read the docs and followed them (if applicable)
• This is not a personal support request that should be posted on the YOURLS Discourse community

Describe the bug
When running yourls in a Docker container, the redirect that takes place after authentication doesn't always work. Sometimes (and I haven't been able to pin this down precisely), the redirect is to http://localhost/admin instead of the correct site URL.

As far as I can tell, this is the only place where the yourls code references $_SERVER[SERVER_NAME] rather than YOURLS_SITE. Making that code change fixes the problem.

To Reproduce

1. Configure docker-yourls to run with YOURLS_SITE set to "http://wibble:8002" and port mapping of 8002:80.

2. Configure Apache to listen on port 8000 and reverse-proxy to http://localhost:8002. This is deliberate because:

   a) we need Apache to be proxying here and

   b) we need Apache to listen on a non-standard port to cause the reply redirection to show as incorrect.

3. Navigate to http://localhost:8000/admin/. Enter valid credentials.

Expected behavior
What should happen is that yourls should return a redirection header of http://wibble:8002/admin/.

Actual behavior
The redirection header is to http://localhost/admin/ instead. Note that there is no port number.

Versions
1.7.6 running inside the yourls Docker container. No extensions or plugins.