chore(renovate): group docker digests

Author: viceice

.github/renovate.json

@@ -79,6 +79,12 @@
       "description": "headlamp cert-manager plugin",
       "matchDepNames": ["headlamp-cert-manager-plugin"],
       "extractVersion": "^cert-manager-(?<version>\\d+\\.\\d+\\.\\d+)$"
+    },
+    {
+      "description": "Group docker digest updates",
+      "matchDatasources": ["docker"],
+      "matchUpdateTypes": ["digest"],
+      "groupName": "docker-digests"
     }
   ],
   "customManagers": [