Allow specifying port through LINKDING_PORT environment variable

[cschmatzler]

While Docker containers have their own IP address namespace and port collisions are impossible to achieve, other container solutions share one.
Podman, for example, runs all containers in a pod under one namespace, which results in every port only being allowed to be assigned once. Having a default port in the image that cannot be changed can cause collisions and incompabilities.

This change allows setting a custom port for UWSGI through the environment while keeping the default port as fallback to keep backwards compatibility.

[sissbruecker]

Thanks, renamed the option to LD_SERVER_PORT to be more consistent with other options and added docs.