Create initial gitlab CI integration
To do continuous integration on pull-requests and enable us to automatically push release artifacts in the future, we require [Gitlab CI/CD pipeline](https://docs.gitlab.com/ee/ci/yaml/README.html) integration.
To start off, we want to build the [default installation medium](https://www.archlinux.org/download/), test it in the two use-cases (BIOS and UEFI based systems) and finally deploy it to an endpoint (if it is a tagged release).
The testing scenario should check for (at least) a successful boot into the install medium (e.g. shell on TTY1 is accessible).
issue