def _convert_volume_from(self, volume_from):
        """
        :param volume_from:
        :return:
        """
        if ':' in volume_from:
            container, permissions = volume_from.split(':')
        else:
            container = volume_from
            permissions = 'rw'

        if permissions not in ('ro', 'rw'):
            raise ValueError("only permissions supported for volumes_from are rw and ro.")

        return "{0}:{1}".format(container, permissions)