code: def _checkout(self):
          
          cmd = ["atomic", "mount", "--storage", "ostree", self.ref_image_name, self.mount_point]
          # self.mount_point has to be created by us
          self._run_and_log(cmd, self.ostree_path,
                            "Failed to mount selected image as an ostree repo.")
docs: check out the image filesystem on self.mount_point