User Namespaces
Network Namespaces
OverlayFS
Rootless OverlayFS is supported since kernel 5.11.
On other distros, Rootless Containers typically use fuse-overlayfs instead of the real OverlayFS.
fuse-overlayfs -o lowerdir=lowerdir/a:lowerdir/b,upperdir=up,workdir=workdir merged