Add regex for matchstring line in Dockerfiles

.github/renovate.json

@@ -16,6 +16,12 @@
       "matchStrings": [
         "#\\srenovate:\\sdatasource=(?<datasource>.*?) depName=(?<depName>.*?)( versioning=(?<versioning>.*?))?( extractVersion=(?<extractVersion>.*?))?\\s.*?_VERSION=(?<currentValue>.*)"
        ]
+    },
+    {
+      "fileMatch": ["(^|/)Dockerfile[^/]*$"],
+      "matchStrings": [
+        "#\\srenovate:\\sdatasource=(?<datasource>.*?) depName=(?<depName>.*?)( versioning=(?<versioning>.*?))?\\s(ENV|ARG) .*?_VERSION=(?<currentValue>.*)\\s"
+       ]
     }
   ]
 }