[docker] skip BE restart in terminating pod

[Al-assad]

Summary

• add a terminating sentinel path for disaggregated BE startup
• exit before any registration/startup side effects when the pod is already marked terminating
• keep the sentinel path overridable via env for operator integration

Test

• bash -n docker/runtime/be/resource/be_disaggregated_entrypoint.sh
• TERMINATING_SENTINEL_PATH=$(mktemp -u) ... minimal local run with a touched sentinel file, verify script exits 0 before start_be.sh

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[hello-stephen]

skip buildall