It runs in a container, so a path like /root exists for the machine and not for
it. The first build in this test works because the hand-started builder runs on
the host; the second is done by the module, and asked it to clone a path it has
no way to reach.
A real module is cloned from the forge over a URL. The lab has no forge, so the
repository goes in the directory the module already mounts — the same fact
wearing different clothes.