renovate: for chainguard (#280)

see dependabot/dependabot-core#2057

Author: v1v

.github/renovate.json

@@ -0,0 +1,6 @@
+{
+    "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+    "extends": [
+        "github>elastic/renovate-config:only-chainguard"
+    ]
+}