android build: persist gradle datadir

avoids re-downloading hundreds of MB of data on every run

electrum/gui/kivy/Readme.md

@@ -42,9 +42,11 @@ folder.
 5. Build binaries
 
     ```
+    $ mkdir --parents $PWD/.buildozer/.gradle
     $ sudo docker run -it --rm \
         --name electrum-android-builder-cont \
         -v $PWD:/home/user/wspace/electrum \
+        -v $PWD/.buildozer/.gradle:/home/user/.gradle \
         -v ~/.keystore:/home/user/.keystore \
         --workdir /home/user/wspace/electrum \
         electrum-android-builder-img \
@@ -76,6 +78,7 @@ $ adb shell monkey -p org.electrum.electrum 1
 ```
 $ sudo docker run -it --rm \
     -v $PWD:/home/user/wspace/electrum \
+    -v $PWD/.buildozer/.gradle:/home/user/.gradle \
     --workdir /home/user/wspace/electrum \
     electrum-android-builder-img
 ```