linux-container: Reset jailed root permissions.

* gnu/build/linux-container.scm (mount-file-systems): Add 'chmod' call.
* tests/containers.scm
("call-with-container, mnt namespace, root permissions"): New test.

gnu/build/linux-container.scm

@@ -170,7 +170,8 @@ for the process."
     (pivot-root root put-old)
     (chdir "/")
     (umount "real-root" MNT_DETACH)
-    (rmdir "real-root")))
+    (rmdir "real-root")
+    (chmod "/" #o755)))
 
 (define* (initialize-user-namespace pid host-uids
                                     #:key (guest-uid 0) (guest-gid 0))

tests/containers.scm

@@ -133,6 +133,14 @@
      (lambda ()
        (primitive-exit 0)))))
 
+(skip-if-unsupported)
+(test-assert "call-with-container, mnt namespace, root permissions"
+  (zero?
+   (call-with-container '()
+     (lambda ()
+       (assert-exit (= #o755 (stat:perms (lstat "/")))))
+     #:namespaces '(user mnt))))
+
 (skip-if-unsupported)
 (test-assert "container-excursion"
   (call-with-temporary-directory