linux-container: Do not jail the container unconditionally.

We may want to run a container inside the MNT namespace, without jailing the
container. If RUN-CONTAINER is passed a null MOUNTS list, do not jail the
container.

* gnu/build/linux-container.scm (run-container): Do not call
MOUNT-FILE-SYSTEMS if MOUNTS list is empty.

gnu/build/linux-container.scm

@@ -243,7 +243,8 @@ (define* (run-container root mounts namespaces host-uids thunk
              (match (read child)
                ('ready
                 (purify-environment)
-                (when (memq 'mnt namespaces)
+                (when (and (not (null? mounts))
+                           (memq 'mnt namespaces))
                   (catch #t
                     (lambda ()
                       (mount-file-systems root mounts