location = /robots.txt {
  add_header  Content-Type  text/plain;
  return 200 "User-agent: *\nDisallow: /\n";
}

[원문]https://alan.ivey.dev/posts/2017/robots.txt-disallow-all-with-nginx/
[참고]https://serverfault.com/questions/549332/how-to-set-robots-txt-globally-in-nginx-for-all-virtual-hosts
[참고]https://edykim.com/ko/post/overwrite-robots.txt-with-nginx-settings/