chore(renovate): group Dockerfile ARG updates separately from Go/NPM

- Replace Go github-tags Dockerfile fallback rule with a generic
  custom.regex + Dockerfile filename matcher
- New group: dockerfile-non-major with "dockerfile" label
- Preserves npm-non-major grouping rule unchanged

Author: actions-user

.github/renovate.json

@@ -414,41 +414,38 @@
       "groupSlug": "go-non-major"
     },
     {
-      "description": "Group Go github-tags fallback updates from Dockerfile custom manager into Go non-major PR",
+      "description": "Group NPM non-major updates into one PR",
       "matchDatasources": [
-        "github-tags"
-      ],
-      "matchManagers": [
-        "custom.regex"
-      ],
-      "matchFileNames": [
-        "Dockerfile"
-      ],
-      "matchPackageNames": [
-        "jackc/pgx"
+        "npm"
       ],
       "matchUpdateTypes": [
         "minor",
         "patch",
         "pin",
         "digest"
       ],
-      "groupName": "go-non-major",
-      "groupSlug": "go-non-major"
+      "groupName": "npm-non-major",
+      "groupSlug": "npm-non-major"
     },
     {
-      "description": "Group NPM non-major updates into one PR",
-      "matchDatasources": [
-        "npm"
+      "description": "Dockerfile ARG trackers (any datasource) group under Dockerfile, not Go/NPM — placed after the datasource group rules so it wins",
+      "matchManagers": [
+        "custom.regex"
+      ],
+      "matchFileNames": [
+        "Dockerfile"
       ],
       "matchUpdateTypes": [
         "minor",
         "patch",
         "pin",
         "digest"
       ],
-      "groupName": "npm-non-major",
-      "groupSlug": "npm-non-major"
+      "groupName": "dockerfile-non-major",
+      "groupSlug": "dockerfile-non-major",
+      "addLabels": [
+        "dockerfile"
+      ]
     },
     {
       "description": "Development branch: Auto-merge non-major updates after proven stable",