Motivation / Background

Ref: rails/bootsnap#495

Fixes #54039. Cross-arch Docker Buildx (QEMU) builds occasionally hang during parallel Bootsnap precompile. Single-threading avoids the deadlock.

Detail

rails/bootsnap#495 (comment)

For those who are investigating, here are my attempts:
...
-j 0 takes around 350 seconds for a small Rails project on my machine, which is not bad compared to the time taken before the hang. (supposedly the time taken if Process.fork works correctly.)
...
The -j 0 option is still the best contender for small projects, IMO.

rails/bootsnap#501

Try to detect QEMU fork bug

Additional information

Lightweight safeguard; no Docker integration test (would be slow/flaky). Can be reverted if upstream issue is fully resolved.

Checklist

• Single focused change.
• Commit message explains rationale and references issue.
• Tests added.
• CHANGELOG (railties) entry to add if deemed user-visible.