Add artifact-registry.yml

This template is to be used alongside the Auto-DevOps template to push the built image to Google Artifact Registry (GAR) in the specific product's meta project (this is being enabled in https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-product-factory/-/issues/70 as part of this work).

Closes #44 (closed)

Related MRs: https://gitlab.developers.cam.ac.uk/uis/devops/devhub/gitlab-runner-infrastructure/-/merge_requests/9 and https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-product-factory/-/merge_requests/73