<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def container_running(self, container_name):
 """ Finds out if a container with name ``container_name`` is running. :return: :class:`Container <docker.models.containers.Container>` if it's running, ``None`` otherwise. :rtype: Optional[docker.models.container.Container] """

        filters = {
            "name": container_name,
            "status": "running",
        }

        for container in self.client.containers.list(filters=filters):
            if container_name == container.name:
                return container
        return None