fix(docker): proper CGO cross-compilation for caire with multi-arch toolchain

Replace CGO_ENABLED=0 (which fails because gioui.org requires CGO on Linux)
with a proper C cross-compiler approach using Debian multi-arch packages.
Running caire-builder with --platform=\$BUILDPLATFORM avoids QEMU crashes on
Apple Silicon; the C cross-compiler bridges the CGO gap for the target arch.

Also adds SKIP_MODEL_DOWNLOADS=true to the CI docker build job to prevent
HuggingFace CDN 504s in CI (image structure is what matters there).

.github/workflows/ci.yml

@@ -93,5 +93,6 @@ jobs:
           file: docker/Dockerfile
           push: false
           tags: ashim:ci
+          build-args: SKIP_MODEL_DOWNLOADS=true
           cache-from: type=gha,scope=unified
           cache-to: type=gha,mode=max,scope=unified