https://github.com/containers/toolbox is a system for integrating containers more closely with a host environment, so that you can switch into a toolbox, automatically give it access to your home directory, and install some compilers.
Can toolbox be used to install Ubuntu on top of a Fedora [Silverblue] host? The default configuration doesn't know about the --distro Ubuntu.
It looks like the toolbox create command has a --image option, so you could try running:
toolbox create --image docker.io/ubuntu:22.10 ubuntu-toolbox
And that almost works, except that toolbox seems to really want the capsh command to be available inside the container. So if we use the following Dockerfile:
FROM docker.io/ubuntu:22.10
RUN apt-get update; apt-get -y install libcap2-bin; apt-get clean
And build an ubuntu-toolbox image:
podman build -t ubuntu-toolbox .
And then create a toolbox:
toolbox create --image ubuntu-toolbox ubuntu-toolbox
We can now enter our toolbox:
$ grep NAME /etc/os-release
NAME="Fedora Linux"
VERSION_CODENAME=""
PRETTY_NAME="Fedora Linux 37 (Workstation Edition)"
CPE_NAME="cpe:/o:fedoraproject:fedora:37"
DEFAULT_HOSTNAME="fedora"
$ toolbox enter ubuntu-toolbox
$ grep NAME /etc/os-release
PRETTY_NAME="Ubuntu 22.10"
NAME="Ubuntu"
VERSION_CODENAME=kinetic
UBUNTU_CODENAME=kinetic