Adopts bash-arithmetic exit-code computation ($((128 + SIGNUM))) instead
of piping to bc, dropping that dependency. Consolidates this project
onto docker-common's shared scripts (see that repo's CLAUDE.md).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NiNnj78HGx1KWyCCo39HSz