enable dependabot for github actions (#1202)

Author: petrjasek

.github/dependabot.yml

@@ -23,3 +23,8 @@ updates:
         versions: [">=6.0.0"]
       - dependency-name: "style-loader"
         versions: [">=2.0.0"]
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"