docs(proposal): add build isolation design for sandboxed builds

[pavank63]

Add design proposal for --build-isolation flag that sandboxes PEP 517 build backend subprocesses using ephemeral Unix users and Linux namespaces. Includes security findings from proof-of-concept testing with build-attack-test package.

See: #1019

• PR follows CONTRIBUTING.md guidelines

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Configuration used: Path: .coderabbit.yaml

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Reviewing files that changed from the base of the PR and between 60e3892 and 5fb0f31.

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• docs/proposals/build-isolation.md
• docs/proposals/index.rst

📝 Walkthrough

Walkthrough

This PR introduces documentation for a build isolation feature proposal. The proposal specifies a --build-isolation/--no-build-isolation CLI flag that sandboxes PEP 517 build backend invocations by creating ephemeral Linux users and isolating network, PID, IPC, and UTS namespaces via unshare. It details environment variable scrubbing, integration points in __main__.py, WorkContext, BuildEnvironment, and external_commands, cleanup procedures via signal handlers, UID mapping, and filesystem setup. The documentation enumerates isolated vs. non-isolated attack surfaces and includes security evaluation based on proof-of-concept testing. A single toctree entry is added to the proposals index to include the new documentation.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding a design proposal document for build isolation feature with sandboxed builds.
Description check	✅ Passed	The description is directly related to the changeset, explaining the build isolation design proposal and its security implications from proof-of-concept testing.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@docs/proposals/build-isolation.md`:
- Around line 206-210: The sentence claiming "Works in unprivileged Podman and
Docker containers" is contradictory because Docker's default seccomp may block
unshare; update the text around that sentence to make Docker support conditional
on seccomp/user namespace configuration: explicitly state that Podman works
unprivileged, and for Docker note that it only works if the container runtime
permits user namespaces/unshare (e.g., using a permissive seccomp profile or
enabling userns), and keep the existing note about Ubuntu 24.04 requiring sysctl
kernel.apparmor_restrict_unprivileged_userns=0; reference the existing terms
"unprivileged Podman and Docker containers", "unshare", and "sysctl
kernel.apparmor_restrict_unprivileged_userns=0" when making the clarification.

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Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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📥 Commits

Reviewing files that changed from the base of the PR and between c833650 and 60e3892.

📒 Files selected for processing (2)

• docs/proposals/build-isolation.md
• docs/proposals/index.rst