Set up Docker test job to allow publishing release images

[riftEmber]

Refactor the Docker nightly testing script to optionally also push release-tagged images.

Configured by the RELEASE_VERSION environment variable, assumed to get set via Jenkins parameter. If set, the script pushes the images tagged as latest and $RELEASE_VERSION. All nightly build/pushes still run before release-tagged pushes, so we don't push a release if any image variants are broken. Script aborts without building anything if RELEASE_VERSION is set but we're not on the appropriate release branch.

As part of this PR I attempted to change our process to test the image before pushing, rather than build, push, then test. However, on chapelmac-m1 this ran into docker/buildx#59, so I reverted it. Since we only push release images after all nightly images, we're still safe from pushing a broken release-tagged image. Noted this limitation and the requirement to build nightly images first in a comment.

Also includes:

• delete unused util/cron/publish-docker-images.bash which didn't contain as much functionality as test-docker.bash
• more comments and some small refactors for clarity

Resolves https://github.com/Cray/chapel-private/issues/6743.

[reviewer info placeholder]

Associated pre-merge tasks:

• merge corresponding CI config adjustments PR https://github.hpe.com/hpe/hpc-chapel-ci-config/pull/1291
• disable unused previous job https://chapel-ci.us.cray.com/job/publish-docker-images/
• update Docker release best-practices to tell you to use this script (https://github.hpe.com/hpe/hpc-chapel-docs/commit/d159f7e8a0be122660cd8af39e3b49ddb8b59486)
• delete temporarily private Docker repositories created for testing (chapel-test{,-gasnet,-gasnet-smp})

Testing:

• manual run in non-release mode still works
• manual run in release mode (temporarily modified to push to a scratch repo)