add dependabot (#7752)

* Create dependabot.yml

* Update dependabot.yml

.github/dependabot.yml

@@ -0,0 +1,18 @@
+version: 2
+updates:
+  # Only care about major updates to github actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    ignore: # Only suggest major updates for built-in actions
+      - dependency-name: "github/*"
+        update-types: ["version-update:semver-patch", "version-update:semver-minor"]
+      - dependency-name: "actions/*"
+        update-types: ["version-update:semver-patch", "version-update:semver-minor"]
+  # Docker dependencies
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+  # no support for c/cpp