Allow forward with the Host header equaling the target server

It appears that the contents of /etc/nginx/conf.d/include/proxy.conf are incorrect and result in the wrong host header being set in proxied requests. The file currently contains:

add_header       X-Served-By $host;
proxy_set_header Host $host;
proxy_set_header X-Forwarded-Scheme $scheme;
proxy_set_header X-Forwarded-Proto  $scheme;
proxy_set_header X-Forwarded-For    $remote_addr;
proxy_pass       $forward_scheme://$server:$port;

It should probably contain:

add_header       X-Served-By $server;
proxy_set_header Host $server;
proxy_set_header X-Forwarded-Scheme $scheme;
proxy_set_header X-Forwarded-Proto  $scheme;
proxy_set_header X-Forwarded-For    $remote_addr;
proxy_pass       $forward_scheme://$server:$port;

The problem is in the first two lines where the variable being used $host which resolves to the SOURCE URL for the proxy rather than the destination which is incorrectly setting the Host header of the proxied request rather than the destination (in $server).

Start with /etc/nginx/conf.d/include/proxy.conf and compare the current proxy header variables with the issue's proposed configuration. Verify the intended destination host behavior for proxied requests, then test the generated Nginx configuration or proxy behavior to confirm that the Host and X-Served-By values use the target server.