ci: add step to navigate to backend directory

.gitea/workflows/ci.yml

@@ -4,6 +4,31 @@ on:
   pull_request:
 
 jobs:
+  checkstyle:
+    name: "Checkstyle Main"
+    runs-on: "ubuntu-latest"
+    container:
+      image: "cimg/openjdk:21.0-node"
+    steps:
+      - name: "Checkout"
+        uses: actions/checkout@v3
+      - name: "Go to backend"
+        run: cd backend
+      - uses: actions/cache@v3
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+      - name: "Prepare Gradle"
+        run: gradle clean
+      - name: "Check"
+        run: gradle check
+      - name: "Stop Gradle"
+        run: gradle --stop
+
   eslint:
     name: eslint
     runs-on: vps-4