Start runner from Docker container instead of systemd service

At present the runner is started as a systemd service. In order to begin moving GitLab pipelines inside Docker, a Docker runner is required. This should be started via a container, with the Docker socket mounted.

More info here: https://docs.gitlab.com/runner/install/docker.html#option-2-use-docker-volumes-to-start-the-runner-container

This will likely need to be added via pillar configuration to be handled by the Docker formula.