Provide more complete build script with download function

[doompadee]

The current build script is a bit lacking in that you have to download the matching gitea binary manually. This should be part of the script.

When running on a DiskStation, the necessary binary can be determined automatically. On other platforms you have to provide the desired package arch or Gitea platform for the download via parameters.

With this feature it's possible to setup a local package server that can be used for fully automated updates or update notification.

If you can agree, I will send a PR. I've redone the script in Python as shell scripting is too awkward.

[flipswitchingmonkey]

I'm not actually using the package at the moment, so I've sent you an invite as a collaborator on the repo, feel free to merge your changes in! Cheers.