Cute, just beware of the problems that might occur if the designated dog is close to the door and another dog walks in. The designated dog walks away and the door closes, locking the other dog in. I would recommend that you have the door hinged at the top and held closed by a spring or similar so that in the event of a power loss, or a dog getting locked in, the dog can just push the door open.
Also you need a very short range RFID reader or the door will never close with the dog inside (or maybe that's the plan).
It actually makes sense then to have the RFID reader some distance outside the door so that the door only lets the dog in, and the dog pushes his way out (that way power failures or faults won't be a learning exercise for him.
The door also needs to stop if there's something in the way, be it a tail, a leg, or bedding. You don't want to hurt the dog or burn out the motor. Maybe the limit switch could also be connected to a bumper on the lower edge of the door.
I have considered something like this for letting our cats and dogs inside the house, or for keeping their food bowls separate (for animals on a special diet).
There would be nothing special about the circuit boards or the coding as nothing has to happen really fast. Limit switches for the door mean that you don't need to be careful at all with the way you control the motors. The only tricky bit is that you may want to keep the door open for a fixed minimum time after the RFID tag is selected so the door opens fully allowing the dog in if he walks too quickly to the opening hatch.
