Migrate build to Google Artifact Registry

[amdw]

We've been asked to stop using the Google Container Registry and move to the Artifact Registry instead.

This looks pretty easy to achieve. I think what's needed is:

• Push a new base container build into the AR and then update the Dockerfile to refer to that
• Update the README.md to no longer refer to gcr.io
• Update the Cloud Build trigger to cause completed builds to be pushed to the AR instead of the GCR

Context: https://cloud.google.com/artifact-registry/docs/transition/transition-from-gcr

[amdw]

I think we're all done here now.