Previously every native build copied the whole linux-$KVER tree from /cache into an ephemeral $BUILD dir, so ccache helped per-file but every link, modpost, kallsyms, and bzImage assembly pass ran fresh. Now the kernel tree lives on a named Docker volume (ac-os-kbuild) mounted at /kernel-build; docker-build.sh seeds it from /cache on first run and keeps the .o files between oven runs. A .kver sentinel wipes the tree when the kernel version bumps. Also: cmp .config against the repo copy before overwriting — the blind `cp` was touching .config's mtime every run, invalidating kbuild's own dependency graph even when the config hadn't changed. Expected win: code-only changes drop from ~10 min kernel rebuild to ~1-2 min (only changed .c files recompile + bzImage re-links with new initramfs). Config changes still do the full rebuild. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>