use local volumes

Add local volumes for caching gradle and android.
This should solve #16.

.drone.yml

@@ -5,6 +5,11 @@ name: default
 steps:
 - name: build 
   image: nextcloudci/android:android-49
+  volumes:
+  - name: gradle
+    path: /root/.gradle
+  - name: android
+    path: /opt/android-sdk-linux
   commands:
   - ./gradlew build
 
@@ -22,3 +27,13 @@ steps:
   when:
     status:
       - failure
+
+volumes:
+- name: gradle
+  host:
+    path:
+      from_secret: volume_gradle
+- name: android
+  host: 
+    path:
+      from_secret: volume_android