Static files getting requested with internal IP address prefix instead of domain-name prefix

I have a smoothly running FreeBSD box with a couple of jails running their own servers.

Nginx runs in its own jail and the main machine uses pf to redirect all http/s requests to Nginx. Nginx checks the requested domain name and does a proxy-pass to the appropriate jail.

This is my first time using Apache/PHP as I just decided to install opencart on a new jail.

The trouble is when I load up the opencart install page, it gets the index.php correctly but then it looks for all the other static files in bad places like this: http://192.168.100.4/install/view/style ... esheet.css - which is basically the internal address of the jail.

I need it to look for the same file, using the domain name instead of the IP address like so: http://shop.foobar.com/install/view/sty ... esheet.css

I am almost (because there are many variables involved) sure this can be done by some php/apache setting, but I haven't been able to figure out how, despite having spent too much time on google. Is the problem due to Apache or something else, and how can I fix it?

A bit further on, I learn that the ip address can be defined in config.php - but after installation. Is there any such equivalent for the install subdir?

Ok I couldn't figure it out in php/apache, so I resorted to using Nginx to pass hostname of the upstream while reverse proxying.