I'm old enough to remember that before docker there was chroot. It's fairly easy to put lots of different user mode portions of Linux distros into directories

I'm old enough to remember that before docker there was chroot. It's fairly easy to put lots of different user mode portions of Linux distros into directories and chroot into them from the same kernel. It seems a bit like what you're asking for. There's also debootstrap which is useful for this technique, not sure if it also works on Ubuntu.