Update the application for modern versions

[mwulffn]

I've updated git-server-docker with the following enhancements:

• Made UID and GID configurable from environment variables (like Linuxserver) - they are now configured by the startup script. Defaults to 1000/1000 as the original image
• Bumped alpine linux version to version 3.22
• Included multiarch building options for both arm and intel platforms.
• Tweaked dockerfile to modern standards
• Minor tweaks to the .gitignore
• Had Claude code update the readme.

The functionality has been preserved and made a little more flexible.