def _extract_tag_value(self, entity, tag_name):
        """Extra tag information from the API result (containers or images).
        Cache extracted tags inside the entity object.
        """
        if tag_name not in TAG_EXTRACTORS:
            self.warning("{0} isn't a supported tag".format(tag_name))
            return

        # Check for already extracted tags
        if "_tag_values" not in entity:
            entity["_tag_values"] = {}

        if tag_name not in entity["_tag_values"]:
            entity["_tag_values"][tag_name] = TAG_EXTRACTORS[tag_name](entity)

        return entity["_tag_values"][tag_name]
Extra tag information from the API result (containers or images).
        Cache extracted tags inside the entity object.