artifact-registry.yml: add option to push feature branch images to artifact registry

After discussions with both Digital Admissions and IAM team members, it has become apparent that devs need the ability to deploy feature branch container images (from their upstream webapp repos) to the development workspace on an adhoc basis via the terraform pipeline which currently is not possible. This issue is to implement a change to the artifact-registry template to allow pushing feature branch images on an adhoc basis to accommodate this request.

The MR also includes a couple of tweaks to the terraform-pipeline which are semi-relevant but need doing 😺