react-native-config/android/dotenv.gradle

def readDotEnv = {
    def env = [:]
    def envFile = System.env['ENVFILE'] ?: "$project.rootDir/../.env"
    new File(envFile).eachLine { line ->
        def (key, val) = line.tokenize('=')
        if (key && val) {
            env.put(key, val)
        }
    }
    env
}

android {
    defaultConfig {
        readDotEnv().each { k, v ->
            buildConfigField "String", k, "\"$v\""
            resValue "string", k, "\"$v\""
        }
    }
}