feat(ci): build multi-arch Docker image (linux/amd64 + linux/arm64) (#137)

Adds QEMU setup and the `platforms: linux/amd64,linux/arm64` arg to the
build-push-action. Apple Silicon Macs and arm64 Linux hosts get a native
image (pulled automatically via manifest selection) instead of running
under QEMU emulation. amd64 users are unaffected.

Tradeoff: arm64 is cross-compiled via QEMU on the x86_64 runner, so
release builds are noticeably longer (the torch install + model bake
stage both run emulated). Acceptable at current release cadence; can
upgrade to a native arm64 runner matrix if it becomes painful.

Author: georgeh0

.github/workflows/release.yml

@@ -126,6 +126,12 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # QEMU lets buildx cross-compile linux/arm64 on the x86_64 runner.
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+        with:
+          platforms: arm64
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -172,6 +178,9 @@ jobs:
           context: .
           file: docker/Dockerfile
           push: true
+          # Native amd64 for Linux servers + Intel Macs; arm64 (cross-compiled
+          # via QEMU) for Apple Silicon Macs and arm64 Linux hosts.
+          platforms: linux/amd64,linux/arm64
           # Install cocoindex-code from the checked-out source tree, not PyPI.
           # Avoids a race where a just-published version hasn't propagated to
           # PyPI's CDN yet (which happened on v0.2.24 release), and ensures