Cache gradle downloads to speed up builds

.gitlab-ci.yml

 image: wkuipers/docker-react-native-yarn:latest
 
+variables:
+    GRADLE_USER_HOME: $CI_PROJECT_DIR/.gradle
+
+cache:
+    key: $CI_PROJECT_NAME
+    paths:
+        - .gradle/wrapper/
+        - .gradle/caches/
+
 before_script:
     - yarn install