ci: Renovate update Rust's Alpine version regex (#148)

Author: DeveloperC286

.github/renovate.json

@@ -31,6 +31,17 @@
       ],
       "datasourceTemplate": "github-tags",
       "depNameTemplate": "earthly/earthly"
+    },
+    {
+      "customType": "regex",
+      "fileMatch": [
+        "^Earthfile$"
+      ],
+      "matchStrings": [
+        "FROM\\s+(?<depName>[\\S]+?):(?<currentValue>[^@\\s]+)(?:@(?<currentDigest>sha256:[a-f0-9]+))?"
+      ],
+      "versioningTemplate": "regex:^(?<compatibility>[\\S]*\\d+\\.\\d+(?:\\.\\d+)?(?:[\\S]*)?-alpine-?)(?<major>\\d+)\\.(?<minor>\\d+)(?:\\.(?<patch>\\d+))?$",
+      "datasourceTemplate": "docker",
     }
   ]
 }