cache build folder.

.gitlab-ci.yml

 android:
   stage: test
   image: jangrewe/gitlab-ci-android:latest
-  before_script:
-    - export GRADLE_USER_HOME=$(pwd)/.gradle
   cache:
-    key: ${CI_PROJECT_ID}
-    paths:
-      - android/.gradle/
+    - key: ${CI_PROJECT_ID} 
+      paths:
+        - build
+    - key: ${CI_PROJECT_ID}
+      paths:
+        - .gradle
   script:
     - echo $KEYSTORE_FILE | base64 -d > my.jks
     - ./gradlew app:assembleRelease