build: Dockerfiles generated by build.py for RHEL require base image specification

[saturley-hall]

What does the PR do?

Images built for RHEL which did not specify a base image defaulted to Ubuntu 24.04 which caused error during the docker build stage as the yum package manager was unavailable. This change causes build.py to error out with a message should the RHEL target be specified without a base image also being specified.

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
• Verified copyright is correct on all changed files.
• Added succinct git squash message before merging ref.
• All template sections are filled out.
• Optional: Additional screenshots for behavior/output changes with before/after.

Commit Type:

Check the conventional commit type
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• build
• ci
• docs
• feat
• fix
• perf
• refactor
• revert
• style
• test

Related PRs:

Where should the reviewer start?

• CI Pipeline ID:

Caveats:

Specification of a RHEL base image does not guarantee that the container will build as there are internal dependencies that may be missing.