docs: def client(self):
          """The original :class:`docker.Client` object. All docker operation
          calling will be forwarded here. ::
  
              docker.create_container('ubuntu')
              docker.client.create_container('ubuntu')  # equivalent
          """
          if not self.app.config['DOCKER_URL']:
              raise RuntimeError('"DOCKER_URL" must be specified')
  
          if not self.app.extensions['docker.client']:
              self.app.extensions['docker.client'] = Client(
                  base_url=self.app.config['DOCKER_URL'],
                  version=self.app.config['DOCKER_VERSION'],
                  timeout=self.app.config['DOCKER_TIMEOUT'],
                  tls=make_tls_config(self.app.config))
          return self.app.extensions['docker.client']