renovate: add custom package rule for pretix calendar versioning

Add custom package rule accounting for pretix calendar versioning to not
have Renovate classify month updates as minor version updates, but major
version updates instead.

renovate.json

@@ -19,6 +19,11 @@
         "minor",
         "patch"
       ]
+    },
+    {
+      "matchDatasources": ["docker"],
+      "matchPackageNames": ["docker.io/pretix/standalone"],
+      "versioning": "regex:^(?<major>\\d+\\.\\d+)(?:\\.(?<minor>\\d+))$"
     }
   ],
   "docker-compose": {