code: def _mount_overlay(self, identifier):
          
  
          cid = self._identifier_as_cid(identifier)
          cinfo = self.client.inspect_container(cid)
  
          ld, ud, wd = '', '', ''
          try:
              ld = cinfo['GraphDriver']['Data']['lowerDir']
              ud = cinfo['GraphDriver']['Data']['upperDir']
              wd = cinfo['GraphDriver']['Data']['workDir']
          except:
              ld, ud, wd = DockerMount._no_gd_api_overlay(cid)
  
          options = ['ro', 'lowerdir=' + ld, 'upperdir=' + ud, 'workdir=' + wd]
          optstring = ','.join(options)
          cmd = ['mount', '-t', 'overlay', '-o', optstring, 'overlay',
                 self.mountpoint]
          status = util.subp(cmd)
          if status.return_code != 0:
              self._cleanup_container(cinfo)
              raise MountError('Failed to mount OverlayFS device.\n%s' %
                               status.stderr.decode(sys.getdefaultencoding()))
  
          return cid
docs: OverlayFS mount backend.