add custom manager for _VERSION variables in dockerfile

renovate.json

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