My 8 month old is the complete opposite. People knock. My two toy dogs bark up a storm. Dexter just yawns and rolls over. I wish he would bark a little when someone is at the door .
Until someone more knowledgeable stops by here's a link to a YouTube video. I get lots of training tips from there. Maybe working it into a silent command when you tell her to be quiet, she stops barking. Hope it helps.
How to Stop Your Dog From Barking at The Door - YouTube