I had a go at this. The core package builds fine, butthere are ~4 "web deps" assets that upstream builds using a whole docker chain. I did not try to understand that, I just used the assets as-built by upstream. This would definitely not be accepted by <whatever ftp-masters is called now>.
Here it is anyway, in case it helps the next person. I probably won't have time to do more work on copyparty.
copyparty_1.20.14-0~PrisonPC1.debian.tar.xz
Description: application/xz
