Another fix.

jail.py

@@ -10,7 +10,7 @@ def jail_run(path, command):
     try:
         res = subprocess.run(command)
     finally:
-        subprocess.run(['umount', os.path.join(path, 'dev')])
+        subprocess.run(['umount', '-f', os.path.join(path, 'dev')])
     if res.returncode != 0:
         # subprocess.run(['umount', os.path.join(path, 'dev')])
         raise RuntimeError('Command failed')