Install a camera if allowed. If the mere threat of being caught on film was not enough then find out when this would typically happen in the night and hire a couple of security guards for a night or two.

Not sure if it would work but not impossible.

Maybe first one could install a bright light, controlled by a motion detector? There are lots of nooks in a city that can be pissed upon in the relative privacy of darkness. If that doesn't do the trick, then a camera could be used, if a sign were posted notifying the public of that use.

Install a urinal nearby?

Why should we accommodate public urination? Just because someone does something, does it automatically mean it's dehumanizing to deter it?

This is what the city can do. What can you do as a shop owner?

Electrifying the metal doors comes to mind, but its illegal, evil, and probably disproportionate to the damage done by the drunkard.

Move the door so that it's flush to the outside wall, rather than providing a niche.

Not doable, it was something like this:

https://www.google.com.br/maps/@-30.0309088,-51.2269515,3a,7...