ci: Try to explicitly setup qemu for arm builds

.github/workflows/push_docker_image.yml

@@ -13,6 +13,8 @@ jobs:
     steps:
       # No checkout step here because build-push-action uses git context by default:
       # https://github.com/marketplace/actions/build-and-push-docker-images#git-context
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
       - name: Login to Docker Hub
@@ -23,7 +25,7 @@ jobs:
       - name: Build and push
         uses: docker/build-push-action@v6
         with:
-          platforms: linux/arm64,linux/amd64
+          platforms: linux/arm64
           push: true
           tags: kinkard/prospero:latest
           cache-from: type=gha