add ci caching

.github/workflows/gradle.yml

@@ -9,16 +9,25 @@ on:
       - master
 jobs:
   build:
-
     runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v2
+      
     - name: Set up JDK 13
       uses: actions/setup-java@v1
       with:
         java-version: 13
+
+    - uses: actions/cache@v1
+      with:
+        path: ~/.gradle/caches
+        key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle') }}
+        restore-keys: |
+          ${{ runner.os }}-gradle-
+      
     - name: Grant execute permission for gradlew
       run: chmod +x gradlew
+  
     - name: Build with Gradle
       run: ./gradlew check