fix: add QEMU and Buildx setup for multi-arch Docker builds

Fixes #290

The workflow was failing because it specified multi-platform builds
(linux/amd64,linux/arm64) but didn't set up QEMU emulation or Buildx.

This adds the required setup steps before the build action.

.github/workflows/build-docker.yml

@@ -17,6 +17,12 @@ 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
+
       - name: Log in to the Container registry
         uses: docker/login-action@v3
         with: