[mikepurvis]

I similarly have built bootable disk images with various tools including buildah and have never been able to fully get away from needing root for various chrooty/loopbacky parts of the process. In principle, it should be very possible to point grub at a filesystem-in-a-file and be like "install to that", but I could never make it happen; it always wanted to be trying to infer things about how to configure itself from examining the host system.

And yes, I've studied the OpenWRT build to no avail. I would be delighted for someone to dissect whatever it is that goes on in there and write it up.