In consideration of the task at hand, I would have assumed a spool of cord, a pulley, and the stepper )or other DC motor, along with a limit sensor- would be the simplest solution, mounted near the top of the door (as most door opening and closing systems are). If you wanted to get funky, force-wise, you could always create a block-and-tackle.
Cord draws door towards wall, pinioned through a pulley (or even an eyebolt). Stepper feeds the cord in and out.