Include ruby dockerfile updates

Author: doshitan

renovate.json

@@ -2,13 +2,11 @@
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": [
     "config:recommended",
+    "customManagers:dockerfileVersions",
     "schedule:monthly",
     ":maintainLockFilesMonthly"
   ],
   "labels": ["dependencies"],
-  "dockerfile": {
-    "enabled": false
-  },
   "github-actions": {
     "fileMatch": ["^template/\\.github/(?:workflows|actions)/.+\\.ya?ml(?:.jinja)?$"]
   },
@@ -31,8 +29,12 @@
         "bundler",
         "ruby-version"
       ],
+      "matchDatasources": [
+        "docker"
+      ],
       "matchDepNames": [
-        "ruby"
+        "ruby",
+        "registry.docker.com/library/ruby"
       ],
       "commitMessageTopic": "Ruby language"
     }

template/{{app_name}}/Dockerfile.jinja

@@ -1,6 +1,7 @@
 # syntax = docker/dockerfile:1
 
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version and Gemfile
+# renovate: datasource=docker depName=registry.docker.com/library/ruby
 ARG RUBY_VERSION=3.3.7