feat: add custom manager to pin build-utils digest

Add a Renovate custom regex manager to detect and pin the digest
of ghcr.io/vexxhost/build-utils in the --mount=type=bind,from=
syntax. This ensures cache invalidation when build-utils is updated.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Mohammed Naser <mnaser@vexxhost.com>

Author: mnaser

renovate.json

@@ -1,4 +1,15 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
-  "extends": ["local>vexxhost/docker-atmosphere"]
+  "extends": ["local>vexxhost/docker-atmosphere"],
+  "customManagers": [
+    {
+      "customType": "regex",
+      "fileMatch": ["(^|/)Dockerfile$"],
+      "matchStrings": [
+        "--mount=type=bind,from=(?<depName>[^:,\\s]+):(?<currentValue>[^@,\\s]+)(@(?<currentDigest>sha256:[a-f0-9]+))?"
+      ],
+      "datasourceTemplate": "docker",
+      "versioningTemplate": "docker"
+    }
+  ]
 }