Add QEMU setup and multi-platform build to the Docker workflow so
the image is published with both linux/amd64 and linux/arm64
support, enabling native execution on Apple Silicon and other
ARM64 hosts.
Set the CLI build version from OBSCURA_VERSION, GitHub tag refs, or Cargo.toml fallback. Use that same build version for both --version and the startup banner, and pass the Docker tag version into Docker builds.
Paul Eipper