[Bugfix] Fix environment variable setting in CPU Dockerfile

[bigPYJ1151]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

After #21343 LD_PRELOAD is not effective in such usage:

docker exec xxx bash -c '...'

It causes the serving test in CPU CI timeouts. This PR fixed this problem with a more robust multi-stage build style.

Test Plan

Test Result

(Optional) Documentation Update

[gemini-code-assist]

Code Review

This pull request correctly addresses the issue of LD_PRELOAD not being set reliably for docker exec commands by switching from modifying .bashrc to using a multi-stage Docker build with ENV instructions. This is a much more robust approach.

My review focuses on a potential security and correctness issue with how the LD_PRELOAD variable is constructed in the new stages. The suggestions aim to make the implementation safer and cleaner by avoiding a leading colon in the environment variable.

[github-actions]

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