fix: add QEMU setup for multi-arch Docker builds

Add docker/setup-qemu-action@v3 before Buildx setup to enable
proper QEMU emulation for linux/arm64 builds on GitHub Actions.

Fixes CI Docker build failure with "execve: No such file or directory"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/docker-build-n8n.yml

@@ -52,6 +52,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3