add renovate config

Add renovate to update the go deps, the cirrus image version and with a
custom manager the core script git sha so we do not lack behind there.
Taken from this example:
https://docs.renovatebot.com/modules/datasource/git-refs/

Fixes #53

Signed-off-by: Paul Holzinger <[email protected]>

.github/renovate.json5

@@ -0,0 +1,50 @@
+/*
+
+Validate this file before committing with (from repository root):
+
+    podman run -it \
+        -v ./.github/renovate.json5:/usr/src/app/renovate.json5:z \
+        ghcr.io/renovatebot/renovate:latest \
+        renovate-config-validator
+
+and/or use the pre-commit hook: https://github.com/renovatebot/pre-commit-hooks
+*/
+
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+
+  /*************************************************
+   ****** Global/general configuration options *****
+   *************************************************/
+
+  // Reuse predefined sets of configuration options to DRY
+  "extends": [
+    // https://github.com/containers/automation/blob/main/renovate/defaults.json5
+    "github>containers/automation//renovate/defaults.json5"
+  ],
+
+  /*************************************************
+   *** Repository-specific configuration options ***
+   *************************************************/
+
+  "ignorePaths": [
+    "**/vendor/**",
+    "**/docs/**",
+  ],
+
+  "addLabels": ["release-note-none"],
+
+  "customManagers": [
+    // Track updates on
+    {
+      "customType": "regex",
+      "fileMatch": "^build.sh$",
+      // Expected version format: gitreporef="xxx"
+      "matchStrings": ["gitreporef=\"(?<currentDigest>.*?)\"\\n"],
+      "currentValueTemplate": "main",
+      "depNameTemplate": "custom-coreos-disk-images",
+      "packageNameTemplate": "https://github.com/coreos/custom-coreos-disk-images",
+      "datasourceTemplate": "git-refs"
+    },
+  ]
+}