refactor(docker): graceful 403 handling

lib/datasource/docker/index.js

@@ -291,7 +291,7 @@ async function getTags(registry, repository) {
       );
       return getTags(registry, 'library/' + repository);
     }
-    if (err.statusCode === 401) {
+    if (err.statusCode === 401 || err.statusCode === 403) {
       logger.info(
         { registry, dockerRepository: repository, err },
         'Not authorised to look up docker tags'