Add docker workflow

.github/workflows/docker.yml

@@ -0,0 +1,35 @@
+name: Build and Push Docker Images
+
+on:
+  push:
+    branches:
+      - docker
+  workflow_dispatch:
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v2
+        with:
+          username: ${{ secrets.DOCKER_HUB_USERNAME }}
+          password: ${{ secrets.DOCKER_HUB_TOKEN }}
+
+      - name: Build and push frontend Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: ./Dockerfile
+          push: true
+          tags: ${{ secrets.DOCKER_HUB_USERNAME }}/njudge:latest
+
+      - name: Logout from Docker Hub
+        run: docker logout