android/app/build.gradle

(...)
signingConfigs {
debug {
  (...)
}

  (...)
  release {
      if (project.hasProperty('RELEASE_STORE_FILE')) {
          storeFile file(RELEASE_STORE_FILE)
          storePassword RELEASE_STORE_PASSWORD
          keyAlias RELEASE_KEY_ALIAS
          keyPassword RELEASE_KEY_PASSWORD
      }
  }
}
buildTypes {
      debug {
          signingConfig signingConfigs.debug
      }
      release {
        signingConfig signingConfigs.release
          (...)
      }
  }
(...)