We’ve tried providing Ubuntu Core images in the past, but found that immutable Linux distros really don’t play well with running inside of containers as they generally require the ability to set up pretty complex filesystem layering right at boot time as well as the ability to swap between versions at boot (and rollback).
That kind of logic is usually handled by the bootloader and/or initrd, neither of which exist in a container world.
So in general, I’d recommend using a VM for such cases.