Replace TARGETARCH build-arg with runtime arch detection via uname -m.
BuildKit's TARGETARCH injection was unreliable for the multi-arch cpu
build, causing the arm64 image variant to download and embed the x86_64
llama-swap binary — resulting in "exec format error" on arm64 hosts.
With QEMU user-space emulation, uname -m correctly returns aarch64
inside an arm64 container build, so the download always fetches the
right binary for the actual target architecture. Also adds --fail to
curl so HTTP 404s produce a build error instead of silently embedding an
HTML error page.
fixes#818
Co-authored-by: Claude <noreply@anthropic.com>
Benson Wong
ccfba0df28