To quickly serve a robots.txt from Nginx without actually having access to the physical file you can define the content of the robots.txt file in the Nginx .conf file.
Allow access to all User-agents:
location /robots.txt {return 200 "User-agent: *\nDisallow:\n";}Disallow access to every User-agent:
location /robots.txt {return 200 "User-agent: *\nDisallow: /\n";}