Support placing user-supplied files in a different directory

Currently, entitlements.json as well as other user-supplied files need to be placed directly in /opt/qt-license-server. For usage in a Docker container it would be useful if files could be easily mounted into the container and live in a dedicated directory, such as /opt/qt-license-server/etc and /opt/qt-license-server/cert (for the SSL certificate), for example.

The current version of the Dockerfile I came up with is attached in case a starting point is needed.