Improve renovate handling of golang version (#20) [skip ci]

Reviewed-on: #20

renovate.json

@@ -12,7 +12,17 @@
           "matchStrings": ["ARG ACT_VERSION=(?<currentValue>.*)"],
           "datasourceTemplate": "gitea-releases",
           "depNameTemplate": "gitea/act_runner"
-        }
+        },
+        {
+          "description": "Manage Alpine OS versions in container image tags",
+          "customType": "regex",
+          "fileMatch": ["Dockerfile$"],
+          "matchStrings": [
+            "FROM\\s+(?<depName>[\\S]+?):(?<currentValue>[^@\\s]+)(?:@(?<currentDigest>sha256:[a-f0-9]+))?"
+          ],
+          "versioningTemplate": "regex:^(?<compatibility>[\\S]*\\d+\\.\\d+(?:\\.\\d+)?(?:[\\S]*)?-alpine-?)(?<major>\\d+)\\.(?<minor>\\d+)(?:\\.(?<patch>\\d+))?$",
+          "datasourceTemplate": "docker"
+        }    
       ],
       "packageRules": [
         {