app gradle file using environment variables.

build.gradle

android {
    signingConfigs {
        release {
            storeFile = file(System.getenv('KEYSTORE_FILE_NAME'))
            storePassword = System.getenv('KEYSTORE_PASSWORD')
            keyAlias = System.getenv('KEYSTORE_ALIAS_NAME')
            keyPassword = System.getenv('KEYSTORE_KEY_PASSWORD')
        }
        debug {
            storeFile = file(System.getenv('KEYSTORE_FILE_NAME'))
            storePassword = System.getenv('KEYSTORE_PASSWORD')
            keyAlias = System.getenv('KEYSTORE_ALIAS_NAME')
            keyPassword = System.getenv('KEYSTORE_KEY_PASSWORD')
        }
    }
    // ...
}