CI: add workflow for building image

.github/workflows/docker-image.yml

@@ -0,0 +1,23 @@
+name: Docker Image CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up JDK 17
+      uses: actions/setup-java@v4
+      with:
+        java-version: '17'
+        distribution: 'temurin'
+        cache: maven
+    - name: Download the backend JAR
+      run: ./download-backend.sh
+    - name: Build the Docker image
+      run: docker build . --file Dockerfile --tag playground-docker:$(date +%s)