ci: add renovate configuration

Keep the pinned GitHub Actions and Dockerfile base images up to date.
Commits follow the repository conventions and are signed off for DCO.

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

renovate.json

@@ -0,0 +1,27 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended",
+    "helpers:pinGitHubActionDigests",
+    ":gitSignOff",
+    ":semanticCommitsDisabled",
+    "schedule:weekly"
+  ],
+  "enabledManagers": ["github-actions", "dockerfile"],
+  "commitMessagePrefix": "ci:",
+  "commitMessageAction": "update",
+  "commitBodyTable": true,
+  "minimumReleaseAge": "7 days",
+  "labels": ["ci", "renovate"],
+  "packageRules": [
+    {
+      "matchManagers": ["github-actions"],
+      "groupName": "GitHub Actions"
+    },
+    {
+      "matchDatasources": ["docker"],
+      "matchDepNames": ["ubuntu"],
+      "allowedVersions": "/^\\d[02468]\\.04$/"
+    }
+  ]
+}