Perform automatic releases

Automatic releases to the official Arch Linux Docker image would be nice.

This requires us to build a GitHub bot to make the PRs.

Maybe @sangy or @shibumi can weigh in here.

Stuff we need to do:

• Create archlinux/official-images (aka fork docker-library/official-images)
• Create an API Key for the technical GitHub User, drop it in a protected variable in GitLab