The resource_group option helps to avoid issues with running multiple
Terraform operations on the same environment at the same time, which
would result in lock errors. See https://docs.gitlab.com/ee/ci/resource_groups/
for more info.

Our GKE runners no longer mount the host's docker.sock due to some permission
issues (see https://gitlab.developers.cam.ac.uk/uis/devops/devhub/gitlab-runner-infrastructure/-/merge_requests/14).
So, this commit fixes this by using docker-in-docker as a service in the CI jobs.

fix(terraform-pipeline): docker issues following changes to GKE runners

See merge request !55