Improve `publish-rhel` script [DI-343]

[JackPGreen]

The action failed but without providing enough information to understand why.

Changes:

• Log the get_image function response (JSON) in case of failure
• Improve failure annotation reporting (as per https://github.com/hazelcast/hazelcast-docker/pull/821)
• Address all Shellcheck warnings in case related
  • Declare and assign separately to avoid masking return values
  • Prefer putting braces around variable references even when not strictly required
  • Quote the rhs of = in [[ ]] to prevent glob matchingQuote the rhs of = in [[ ]] to prevent glob matching

Partially addresses DI-343

Post-merge actions:

• [ ] backport

[JackPGreen]

Have you tested it?

I hadn't - because it's destructive I wanted to wait for a review.

As it's already had one approval, I've run it here ✅.

Unless you could think of a better way to test?