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 integration.

To start off, we want to build the default installation medium, 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).