Not willing to review, as I am not willing to maintain this. #239 (comment)

So how is anyone supposed to know how to build? E.g., a distribution that wants to ship libappimageupdate?

They can read the Dockerfile, similar to how you did. Your information is not useful anyway as it focuses on an almost EOL Ubuntu and uses PPAs. It's not hard to make a package. A packager can figure it out in half an hour, worst case.

Edit: following the agile manifest, this is pointless, too: nobody ever asked for this information. If they did, we could help them. But right now, this is not useful to anybody. It's a waste of time.