feat(renovate): add customManager to update `_VERSION` variables from Dockerfiles

renovate.json

@@ -1,3 +1,16 @@
 {
-    "$schema": "https://docs.renovatebot.com/renovate-schema.json"
+    "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+    "customManagers": [
+      {
+        "customType": "regex",
+        "description": "Update _VERSION variables in Dockerfiles",
+        "fileMatch": [
+          "(^|/|\\.)Dockerfile$",
+          "(^|/)Dockerfile\\.[^/]*$"
+        ],
+        "matchStrings": [
+          "# renovate: datasource=(?<datasource>[a-z-]+?)(?: depName=(?<depName>.+?))? packageName=(?<packageName>.+?)(?: versioning=(?<versioning>[a-z-]+?))?\\s(?:ENV|ARG) .+?_VERSION=(?<currentValue>.+?)\\s"
+        ]
+      }
+    ]
 }