Use balenalib/raspberry-pi-debian:stretch with legacy.raspbian.org repos.
Exact match: GCC 6.3, Python 3.5, GLIBC 2.24 — identical to the Pi.
First build ~25min (QEMU), subsequent builds ~1sec (cached image).
Replaces the broken Bullseye approach that had GLIBC/GLIBCXX mismatches.