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

Our GKE runners no longer mount the host's docker.sock due to some permission issues (see uis/devops/devhub/gitlab-runner-infrastructure!14). This MR fixes this by using docker-in-docker as a service in the terraform-pipeline CI jobs.

As an additional enhancement, this MR configures the resource_group option for terraform-pipeline CI jobs. This helps to avoid issues with multiple Terraform operations executing concurrently for the same environment, which would result in lock errors. See https://docs.gitlab.com/ee/ci/resource_groups/ for more info (this is also how the GitLab official Terraform templates are configured).

Part of #47 (closed) & #50 (closed)