I’ve run into this problem too. Another solution is to toggle the SELinux boolean value for httpd network connect to on
(Nginx uses the httpd label).
setsebool httpd_can_network_connect on
To make the change persist use the -P flag.
setsebool httpd_can_network_connect on -P
You can see a list of all available SELinux booleans for httpd using
getsebool -a | grep httpd