Configure Renovate to auto update chainguard image (#114172)

* Adds renovate.json
* Only auto update chainguard image
---------

Co-authored-by: elastic-renovate-prod[bot] <174716857+elastic-renovate-prod[bot]@users.noreply.github.com>
Co-authored-by: Rene Groeschke <[email protected]>

Author: elastic-renovate-prod[bot]

renovate.json

@@ -0,0 +1,21 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "github>elastic/renovate-config:only-chainguard"
+  ],
+  "customManagers": [
+    {
+      "description": "Extract Wolfi images from elasticsearch DockerBase configuration",
+      "customType": "regex",
+      "fileMatch": [
+        "build\\-tools\\-internal\\/src\\/main\\/java\\/org\\/elasticsearch\\/gradle\\/internal\\/DockerBase\\.java$"
+      ],
+      "matchStrings": [
+        "\\s*\"?(?<depName>[^\\s:@\"]+)(?::(?<currentValue>[-a-zA-Z0-9.]+))?(?:@(?<currentDigest>sha256:[a-zA-Z0-9]+))?\"?"
+      ],
+      "currentValueTemplate": "{{#if currentValue}}{{{currentValue}}}{{else}}latest{{/if}}",
+      "autoReplaceStringTemplate": "\"{{{depName}}}{{#if newValue}}:{{{newValue}}}{{/if}}{{#if newDigest}}@{{{newDigest}}}{{/if}}\"",
+      "datasourceTemplate": "docker"
+    }
+  ]
+}