Thank you Francis for pointing the right direction.

If I understand correctly the documentation I should add
proxy_set_header Host $host;

considering I had the upstream server defined by: upstream webservers { server virt1:80; }

I tried it but had no luck with it, I also tried $http_host; and $proxy_host;

Also should I place this proxy_set_header before the proxy_pass ? I think yes, could you confirm this ?

Regards