docker scratch: return None for parent

so we don't query docker for no reason

Author: TomasTomecek

sen/docker_backend.py

@@ -384,6 +384,10 @@ def __init__(self, docker_backend):
         self.image_name = "scratch"
         super().__init__(None, docker_backend, object_id="")
 
+    @property
+    def parent_id(self):
+        return None
+
     @property
     def parent_image(self):
         return None