💚 Backend の CI を追加

.github/workflows/backend.yml

@@ -0,0 +1,31 @@
+name: "Backend CI"
+
+on:
+  pull_request_target:
+
+# Cancel any current or previous job from the same PR
+concurrency:
+  group: backend-${{ github.head_ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          lfs: 'true'
+
+      - uses: actions/setup-java@v3
+        with:
+          distribution: 'zulu'
+          java-version: 17
+
+      - name: Setup Gradle
+        uses: gradle/gradle-build-action@v2
+
+      - name: Unit Tests
+        run: ./gradlew :app:backend:check
+
+      - name: Build
+        run: ./gradlew :app:backend:buildFatJar