react-native-config/android/dotenv.gradle

def readDotEnv = {
    def env = [:]
    def envFile = System.env['ENVFILE'] ?: ".env"
    println("Reading env from: $envFile")
    try {
      new File("$project.rootDir/../$envFile").eachLine { line ->
          def (key, val) = line.tokenize('=')
          if (key && val && key.substring(0, 1) != "#") {
              env.put(key, val)
          }
      }
    } catch (Exception ex) {
      println("**************************")
      println("*** Missing .env file ****")
      println("**************************")
    }
    project.ext.set("env", env)
}

readDotEnv()

android {
    defaultConfig {
        project.env.each { k, v ->
            def escaped = v.replaceAll("%","\\\\u0025")
            buildConfigField "String", k, "\"$v\""
            resValue "string", k, "\"$escaped\""
        }
    }
}