Add dependabot for github actions (#16)

* Add dependabot for github actions

Add [dependabot](https://github.blog/2020-06-01-keep-all-your-packages-up-to-date-with-dependabot/) configuration for github actions.

* Add gitsubmodules to dependabot

* Set default reviewers and labels for dependabot

* Remove submodules check 

Ref: https://github.com/docker-mailserver/docker-mailserver/pull/16#issuecomment-762086895

.github/dependabot.yml

@@ -0,0 +1,13 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    reviewers:
+      - "radicand"
+      - "aendeavor"
+      - "wernerfred"
+    labels:
+      - "area/dependency"
+      - "pr/needs review"