refactor(docker): reduce cache time to 15 mins

lib/datasource/docker.js

@@ -230,7 +230,7 @@ async function getTags(registry, repository) {
     } while (url && page < 20);
     logger.debug({ length: tags.length }, 'Got docker tags');
     logger.trace({ tags });
-    const cacheMinutes = 30;
+    const cacheMinutes = 15;
     await renovateCache.set(cacheNamespace, cacheKey, tags, cacheMinutes);
     return tags;
   } catch (err) /* istanbul ignore next */ {