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/stylesheet/stylesheet.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/stylesheet/stylesheet.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 (pf?), and how can I fix it? Given the annoying nature of this problem, clues might be just as useful as answers.