docker/setup-qemu-action registers binfmt via a privileged side container;
pi-gen-action's own nested Docker container doesn't inherit the
registration. Result: arm64 ELFs in the pi-gen chroot still fail to
exec, exit 1 before any stage runs.
apt-installed qemu-user-static + binfmt-support writes persistent
binfmt_misc entries to the kernel that propagate to every container
share. Pair with update-binfmts --enable qemu-aarch64 and a sanity
ls -la /proc/sys/fs/binfmt_misc/qemu-aarch64.