Extract prepare signing step

.drone.yml

@@ -4,22 +4,23 @@ type: docker
 name: Android
 
 steps:
-- name: build
+- name: prepareSigning
-  image: mingc/android-build-box
+  image: busybox
   environment:
     STOREPASSWORD:
       from_secret: StorePassword
     KEYPASSWORD:
       from_secret: KeyPassword
-  volumes:
-  - name: deploy
-    path: /tmp/deploy
   commands:
   - touch keystore.properties
   - echo "storePassword=$STOREPASSWORD" >> keystore.properties
   - echo "keyPassword=$KEYPASSWORD" >> keystore.properties
   - echo "keyAlias=key0" >> keystore.properties
   - echo "storeFile=../AndroidKey" >> keystore.properties
+
+- name: build
+  image: mingc/android-build-box
+  commands:
   - ./gradlew test
   - ./gradlew assembleDebug
   - ./gradlew bundleRelease