code: async def was_killed(self, container_id):
          
          if container_id in self._watching:
              self._watching.remove(container_id)
          if container_id in self._container_had_error:
              self._container_had_error.remove(container_id)
              return "timeout"
          return None
docs: This method has to be called *once, and only once* for each container registered in `register_container`.
          :param container_id: the container id to check
          :return: a string containing "timeout" if the container was killed. None if it was not (std format for container errors)