build.gradle

android {

    signingConfigs {
        release
    }

    buildTypes {
            release {
                signingConfig signingConfigs.release
            }
    }
}

def Properties props = new Properties()
def propFile = new File('signing.properties')
if (propFile.canRead()){
    props.load(new FileInputStream(propFile))

    if (props!=null && props.containsKey('STORE_FILE') && props.containsKey('STORE_PASSWORD') &&
            props.containsKey('KEY_ALIAS') && props.containsKey('KEY_PASSWORD')) {
        android.signingConfigs.release.storeFile = file(props['STORE_FILE'])
        android.signingConfigs.release.storePassword = props['STORE_PASSWORD']
        android.signingConfigs.release.keyAlias = props['KEY_ALIAS']
        android.signingConfigs.release.keyPassword = props['KEY_PASSWORD']
    } else {
        println 'signing.properties found but some entries are missing'
        android.buildTypes.release.signingConfig = null
    }
}else {
    println 'signing.properties not found'
    android.buildTypes.release.signingConfig = null
}

signing.properties

STORE_FILE=/path/to/your.keystore
STORE_PASSWORD=yourkeystorepass
KEY_ALIAS=projectkeyalias
KEY_PASSWORD=keyaliaspassword