Neighbor's dog attacked my stray cat. What should I do?

Sorry, this is a long post…!

I've been taking care of a stray cat for about six months now. I can't take him in because I'm not allowed pets in my condo, and he gets scared whenever I shut him inside.

Anyway, this afternoon, my neighbor's dog attacked the cat when it was climbing our shared stairs to come get food from me. It tore a big chunk of the cat's hair out to the left of his mouth. I see pink skin and two smallish red spots that look like they're open wounds, both maybe the size of a marker tip. First question: I tried to put Neosporin on the wounds, but when I put the tip of the tube near its wound, it moves its head. Any ideas on how I can get it on?

Second, what should I do about the neighbor's dog? Someone told me to report them to the police, but that seems a bit extreme. However, one time the dog got loose in a windstorm and tried to bite both me and another neighbor's husband.

The dog's runner is attached to the owner's gate, but they put him on a runner that must let the dog go down their stairs onto our shared stairs, because that's where the cat was attacked.

I am worried the dog can get a few feet down our split stairs, which means it could eat the cat or try to attack me or someone else.

I know I should talk to the dog owners but I am bad at doing that sort of thing without sounding accusatory. Also, I don't know if I should tell the neighbors I have been feeding a stray, because they will probably get mad and report me to animal control or the condo association.

What should I do?!