Paintless dent repair, very popular on expensive cars over here in Germany and in other countries like the US.
You won't believe what these guys can do, you won't see a thing afterwards. This only works if the dent didn't split the paint and it may not work in certain areas where the body has weird shapes. Door? Shouldn't be any problem.
I'm sure there is a good "dent doctor" in your area in HK too, just ask around.