[fix](docker) Propagate BE registration failures

[Al-assad]

What problem does this PR solve?

Issue Number: N/A

Related PR: N/A

Problem Summary:

The Kubernetes BE entrypoint scripts initialized REGISTERED to the literal string false and later evaluated it with [[ $REGISTERED ]]. In Bash, every non-empty string is true, so an FE registration failure could be treated as success and the script would continue to start BE without registering it with FE.

This PR removes the unused string flag and uses the exit status from add_self / first_deploy_start directly. Each configured FE address is tried in order. If all registration attempts fail, the entrypoint exits before start_be.sh, allowing Kubernetes to restart the container and retry after FE becomes available.

Supersedes #66922, which GitHub automatically closed after its cross-fork head branch was renamed.

Release note

Fix Kubernetes BE containers to restart instead of starting unregistered when FE registration fails.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
    • bash -n docker/runtime/be/resource/be_entrypoint.sh
    • bash -n docker/runtime/be/resource/be_disaggregated_entrypoint.sh
    • build-support/shell-check.sh
    • Mocked registration success, first-address failover, and all-addresses-failed return paths for both scripts
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes. BE containers now exit when all FE registration attempts fail instead of starting unregistered.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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[Al-assad]

/review

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