Werk #11086: Docker container: Fix usage behind reverse proxies

The Checkmk container was not operating as expected in different scenarios where it's user interface was accessed through a reverse proxy, for example when it was working behind a K8s load balancer.

This behavior now has been improved by removing the port from the "ServerName" directive in the docker container. This makes the web server of the container detect and use the port from client requests and reuses it to build URLs for redirects and so on.

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