GitHub Actions: cache gradle dependencies

.github/workflows/ci.yml

@@ -19,5 +19,11 @@ jobs:
       uses: actions/setup-java@v1
       with:
         java-version: 1.8
+    - name: Cache Gradle packages
+      uses: actions/cache@v2
+      with:
+        path: ~/.gradle/caches
+        key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle') }}
+        restore-keys: ${{ runner.os }}-gradle
     - name: Build with Gradle
       run: ./gradlew build