ci: new jobs image publishing configuration

[grayside]

Changes in this PR

• Adds support for hello-job image tag
• Renames build config "merge" (a triggering model) to "publish" (the purpose of the config)
• Updates for easier-to-read script syntax
• Switches to a pre-built pack container image

For comparison, see https://github.com/GoogleCloudPlatform/cloud-run-hello/blob/master/testing/jobs.merge.cloudbuild.yaml

Why a new file?

This change keeps the existing build config during a transition period, and comments the file to that purpose.

Why multiple changes in one PR

If someone wants I can break it up, but before arriving at this simple set of changes, I explored if this configuration should be made generic across the repository. Landing back on looks like several small changes but in fact is a re-simplification from a significant overhaul.

[iennae]

LGTM; thanks for the detailed description of what is intended by the changes proposed.