Consider adding /run/host/lower-root

[cgwalters]

When using transient root, tooling (or an admin interactively) may want to be able to scan and detect changes in the overlayfs upper versus the lower.

systemd has a container-to-host API The /run/host/ Hierarchy set defined; we could propose adding a /run/host/lower-root there that is a mount point (or symlink potentially) to the merged lower.

Then an admin or tools can just diff the two filesystem trees however they want (e.g. diff -u /run/host/lower-root/usr /usr).