`docker/macos` builds for aarch64-apple-darwin from Linux with
cargo-zigbuild. Zig carries libSystem and the C headers, so tract's SIMD
kernels compile and the engine's test binaries and examples link as Mach-O
arm64 — the check `cargo check --target` could not do, because tract's
build script needs a macOS C compiler. Crates that link an Apple framework
(dr-plat's keyring, and so the app) still need the Xcode SDK and fail at
the link; macos.md says so.
