Consider publishing all external test images to Github packages

In the past, we included the image generation during the test pipeline because we were supporting an extremely large number of variations / tags.

Since we have trimmed back the number of variants we support, we could consider building and publishing these images to ghcr.io when material inside of the external directory changes, and not during every test pipeline run.

This issue is to investigate the number of images we would want to publish (perhaps all of sanity.external ones that get run during EA pipelines), look at creating a workflow that builds and publishes them, and updates in our test pipeline code to look for an image and pull from the ghcr.io registry if its present, otherwise fall back to building the image from scratch versus always building from scratch (determine how much execution time is saved, the ease or complexity of such a change).

Once we gather some of the above information, we can make a decision on right next steps.

Helpful [doc](https://docs.github.com/en/actions/use-cases-and-examples/publishing-packages/publishing-docker-images#publishing-images-to-github-packages) for pushing to ghcr.io from a workflow.

Related: https://github.com/adoptium/aqa-tests/issues/5657#issuecomment-2484465545

Start by reviewing the images under the external directory, especially the sanity.external images run during EA pipelines, and the existing test pipeline code that builds them. Read the linked GitHub Packages workflow documentation and measure the potential execution-time savings. Done means documenting the image inventory, workflow complexity, fallback behavior, and enough information to decide next steps.

Written by the indexing model from the issue text.