Add dependabot for github actions (#16)
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
tomav/docker-mailserver,tomav/docker-mailserver,tomav/docker-mailserver