Refactor CI workflow to use Docker Buildx for building and pushing images

.gitea/workflows/ci.yaml

@@ -25,21 +25,25 @@ jobs:
             --username ${{ secrets.REGISTRY_USERNAME }} \
             --password-stdin
 
+      - name: Set up Docker Buildx
+        run: |
+          docker buildx create --use --name multiarch || docker buildx use multiarch
+
       - name: Build and push backend
         run: |
-          docker build \
-            --platform linux/amd64 \
+          docker buildx build \
+            --platform linux/amd64,linux/arm64 \
             -t gitea.home.arpa/nik/watch-party-backend:latest \
+            --push \
             /tmp/watch-party/backend
-          docker push gitea.home.arpa/nik/watch-party-backend:latest
 
       - name: Build and push frontend
         run: |
-          docker build \
-            --platform linux/amd64 \
+          docker buildx build \
+            --platform linux/amd64,linux/arm64 \
             -t gitea.home.arpa/nik/watch-party-frontend:latest \
+            --push \
             /tmp/watch-party/frontend
-          docker push gitea.home.arpa/nik/watch-party-frontend:latest
 
       - name: Deploy to Mac Mini
         run: |