[#62996016] Update nginx.conf template for ipv6only=off

 - nginx >=1.3.4 defaults to ipv6only=on for listening ipv6 sockets (whereas previously it would default to os setting)
 - sets ipv6only=off to keep wildcard address listening on both ipv4 and ipv6 addresses
Ensure generate_nginx_conf picks up RAILS_ENV value