CI: switch to docker buildx and enable arm64 docker images

.github/workflows/build-docker.yml

@@ -14,5 +14,13 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
-      - name: Build the Docker image
-        run: docker build . --file Dockerfile --tag rofl256/whiteboard:$(date +%s)
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: build the image
+        run: |
+          docker buildx build \
+          --file Dockerfile \
+          --tag rofl256/whiteboard:$(date +%s) \
+          --platform linux/amd64,linux/arm64 .