This is a follow-up of 5316dfc0f125b658e4a2acf7f00f49501663d943. Some users of
run-container may expect that the container is jailed, even if there are no
mounts. This is the case for some Guix tests.
* gnu/build/linux-container.scm (run-container): Do not jail the container
when the requested root is "/".