Add a `mounts` placement option to the guest builder: a Host maps guest
interior paths to host paths, each read-write by default or read-only per
mount, realized as the nested container's bind mounts. A guest sees exactly the
data it should, at any granularity from a single folder to a whole pool.
Pin the container to the host's uid and gid space with `privateUsers = "no"`,
so a guest process writing as the shared storage group lands on a bind-mounted
pool as that same group without permission juggling.
alexion
ab9b9e9f8f