def sort_annotations(self):
        """
        Return a map of maps to look up a single "worker digest" that has information
        about where to find an image manifest for each registry/architecture combination:

          worker_digest = <result>[registry][architecture]
        """

        all_annotations = self.workflow.build_result.annotations['worker-builds']
        all_platforms = set(all_annotations)
        if len(all_platforms) == 0:
            raise RuntimeError("No worker builds found, cannot group them")

        sorted_digests = {}

        for plat, annotation in all_annotations.items():
            for digest in annotation['digests']:
                hostname = registry_hostname(digest['registry'])
                media_type = get_manifest_media_type(digest['version'])
                if media_type not in self.manifest_media_types:
                    continue

                platforms = sorted_digests.setdefault(hostname, {})
                repos = platforms.setdefault(plat, [])
                repos.append(digest)

        sources = {}
        for registry in self.registries:
            registry_conf = self.registries[registry]
            if registry_conf.get('version') == 'v1':
                continue

            hostname = registry_hostname(registry)
            platforms = sorted_digests.get(hostname, {})

            if set(platforms) != all_platforms:
                raise RuntimeError("Missing platforms for registry {}: found {}, expected {}"
                                   .format(registry, sorted(platforms), sorted(all_platforms)))

            selected_digests = {}
            for p, repos in platforms.items():
                selected_digests[p] = sorted(repos, key=lambda d: d['repository'])[0]

            sources[registry] = selected_digests

        return sources
Return a map of maps to look up a single "worker digest" that has information
        about where to find an image manifest for each registry/architecture combination:

          worker_digest = <result>[registry][architecture]