you do not understand... Not run on, run IN :)

I'm saying the older version of macos could build/run INSIDE the container

just like on a ubuntu 24.04 system you can do:

  FROM ubuntu:16.04

or

  docker run ubuntu:16.04 

and though I haven't tried it, I believe docker can do arm in x86 using an emulator (like rosetta)

You can already run older versions of macOS inside a VM on macOS.

So it seems like in theory that should be doable if someone just made the container images right?

Oh right. I don't see Apple having any interest in supporting that given their current trajectory.