The runc backend doesn't work with unified cgroup hierarchy, and it
failing is a known issue.
However, the default backends should work in both rootless and as-root
scenarios, so make sure we test these.
This is required by (among others) Podman to run containers in rootless mode.
Other distributions such as Fedora and Ubuntu already set up these mappings.
The scheme with a start UID/GID offset starting at 100000 and increasing in 65536 increments is copied from Fedora.
