Nginx request port
Solution 1:
Hopefully not too little too (eight months!) late. I had a similar question myself, to use the original request port in nginx.conf.
nginx $http_name variable
$http_name: arbitrary request header field; the last part of a variable name is the field name converted to lower case with dashes replaced by underscores
$http_host should therefore contain the request 'Host' header, if that helps.
e.g. localhost:8020