Dockerfile now uses BuildKit TARGETARCH/TARGETVARIANT to pick the Rust
cross-compilation target automatically. The build stage always runs on
the host platform for speed.
Makefile provides named targets:
make up-amd64 # Mac Intel / Linux desktop
make up-arm64 # Mac M1/M2/M3, Pi 4/5 (64-bit OS)
make up-armv7 # Pi 2/3/4 (32-bit OS)
make up-armv6 # Pi Zero / Pi 1