To align better with the [guidebook documentation][1] for using docker
in docker for CI jobs, add the optional-but-recommended `before_script`
to the fragment.

Closes #178

[1]: https://guidebook.devops.uis.cam.ac.uk/en/latest/explanations/cloud-platform/gke-gitlab-runners/#using-docker-in-ci-jobs

feat(docker-in-docker): add waiting before_script to fragment

Closes #178

See merge request !218