Backport of #60742.
This PR resurrects support for building Docker images based on one of
Red Hat's UBI images. It also adds support for running the existing
Docker tests against the image. The image is named
`elasticsearch-ubi8:<version>`.
I also changed the Docker build file uses enums instead strings in a lot
of places, for added rigour.
This commit registers the UBI-based Docker image projects in the build
so that their assemble tasks are executed when the top-level assemble
task is executed.
