Fix host /dev/pts unmount during overlayfs cleanup#9

Draft

Copilot wants to merge 10 commits intobind-003from

copilot/docker-ubuntu-overlayfs-test

Commits on Dec 4, 2025

Initial plan
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Fix devpts unmount issue: use safe unmount without MNT_DETACH

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LangQi99
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Add unit tests and comprehensive documentation for devpts fix

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Add comprehensive technical documentation and testing guides

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LangQi99
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Address code review feedback: clarify comments and improve logging

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LangQi99
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Security improvements: fix path traversal vulnerability and clarify behavior

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Strengthen security: abort on canonicalization failure, improve error handling

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Code polish: extract helper function, improve variable names, fix docs

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Final refinement: use custom enum for clearer API and improve security logging

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LangQi99
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Add comprehensive PR summary document

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LangQi99
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