Document use and release process for `hyrax-builder` images

[hannahilea]

This code exists in utils/ directory; there's a README there that is rh9-specific.

As of #99, the images will be built and snapshots tagged from CI; after #100, there will be scheduled snapshots depoyed.

The release process (testing and tagging a "latest" version) is and will be manual, for at least the forseeable future.

The steps for testing and making a released latest image should be documented somewhere (presumably, in this hyrax-docker repo, at either the top level README or the utils/README).