differentiate between release and debug builds

Building with KEYSTORE_PASSWORD and KEYSTORE_URL defined in the environment will initiate a release build signed with the keystore pulled from KEYSTORE_URL which is unlocked by the password in KEYSTORE_PASSWORD. After signing, the keystore file is immediately deleted.  
In practice this would look something like this:  
__ssh into build server__  
`sudo docker run --env KEYSTORE_PASSWORD=somepassword --env KEYSTORE_URL=http://www.google.com libretro/android-builder build-now.sh android_all`  
`sudo docker cp $(docker ps -l -q):/staging/ .`  
Now the release .apk will be in a directory called staging in the current working directory