nginx/Dockerfile.template: Change default hostname to _

nginx/Dockerfile.template: Change default hostname to _

From the docs [1]:

  In catch-all server examples the strange name “_” can be seen:

    server {
        listen       80  default_server;
        server_name  _;
        return       444;
    }

  There is nothing special about this name, it is just one of a myriad
  of invalid domain names which never intersect with any real name.

Still, an invalid domain name makes more sense than 'localhost' for a
public server.

[1]: http://nginx.org/en/docs/http/server_names.html#miscellaneous_names