Fix jail permission setup for non-root runners - #106
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A non-root microVM runner with the documented capability set could not prepare its jail: ownership moved to the VM uid before
chmod, which then failed withEPERM. Reclaim the inode withCAP_CHOWN, set its mode while owned by the runner, and transfer it to the VM. The same sequence handles an inode retained across resume without addingCAP_FOWNER. An engine ownership mutex keeps concurrent launches from interleaving the sequence through hard links to the same home inode.A real Linux regression launches a non-root child with only
CAP_CHOWNand checks initial setup, repeated ownership transfer, and concurrent shared-inode transfers. It failed before the fix and passes after it; Linux CI now runs it explicitly. Focused jail tests pass, the Linux concurrency regression passed ten repetitions, and full CI passed for commit 2983175 (make verify, restricted-capability regression, and CLI/client race checks). Both independent reviews pass after fixing the adversarial concurrency finding.The privileges document records a separate unresolved limitation: jailer 1.17.0 writes ancestor cgroup controls outside a delegated subtree. This PR fixes inode ownership; it does not qualify the complete non-root launch path or change the capability set.