Add a workflow to push to Docker hub on release

[ChrisLovering]

This pushes an image to dockerhub with the tags latest, a short git sha, and the tag of the release.

The trigger for this workflow is when a release is marked as released.

This workflows requires two secrets to be made available to the repo:

• DOCKERHUB_USERNAME the username to login as
• DOCKERHUB_TOKEN a personal access token with access to the modmail image