I think if it's only gonna be for short periods of time, you don't have to worry about heating it at all. All ourdogs had their way of saying "let me in, I'm done being outside."

My Sheba would come to the kitchen door, sit politely and give the nicest "please" bark. Just a quietish single "bark!"

Freckles, a St. Bernard, would paw at the (metal-framed) screen door, which made enough of a racket so someone would respond instantly.

Gracie would just curl up in a sunny spot and wait (being black and shiny, she was a solar-powered doggie) or if she got tangled with her chain around something, she'd "boof" pitifully from the yard!

I wouldn't worry about it, it's nice that you want to give your future dog a sheltered spot, and with the nice dense coat a GSD has, a doghouse will be plenty warm for a couple hours.