Speed upGitlab CI pipeline by using prebuilt Docker image

Squashed commit of the following:

commit 270880c1a815a8652407bd334ffde1e775845c5f
Author: Daniel Friesel <daniel.friesel@uos.de>
Date:   Thu Aug 13 11:05:24 2020 +0200

    herp derp

commit e0546c27220b9afcd6c6bca544fab8d6d816f61f
Author: Daniel Friesel <daniel.friesel@uos.de>
Date:   Thu Aug 13 10:57:59 2020 +0200

    gitlab CI: Use multipass base image, do not pull in recommended packages