I have created a Docker container based on the changes from the “moddwarf-new build change” in my GitHub fork. It does not include libgig, as I originally described. Anyone can test it now.
I guess that this is a result of the complex game of upgrading without breaking things, upgrading without having the time and resources to do a massive upgrade, or maybe there was no point for the devs to change what was OK. Until it wasn’t. We may not be the right people to give you the reasons why.