The current AppImage docker image is broken. It's still broken in go-flutter-desktop/hover#73 and I couldn't find a fix for it or even anyone on the internet that has the same problem.
It fails with this message:
/bin/sh: 1: ./appimagetool-x86_64.AppImage: not found
after downloading and chmod'ing it. So the problem is not that the file doesn't exists, but somehow can't be executed.