Update runner doctor catalog for ARC/DinD DNS, gVisor MCP 403s, and sbx home exposure (#6409)

* Initial plan

* docs: update runner doctor failure mode catalog

* docs: address runner doctor review feedback

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot