I do not envy you folks with garage shops. We, luckily, have a home with a big basement that is partitioned off with several rooms, one of which has a garage door and my fun car's shop. Adjacent and in the next room is my wood shop. The room with that garage door - the opening needs to be closed off late every fall as the room gets to meat locker temp if I don't. The panels are a simple 2x4 framing and a painted-to-match-the-house luan ply exterior. These panels are insulated with 1-1/2" styrofoam insulation (I just bought another 1-1/2" layer to add one of these days. I also use foam tape around the edges to seal the small gaps that the builder and time have given me on this 1970 vintage house. When the panels are in, down come the door - and it gets as warm as I want it (usually 64*F) as the basement as well as the rest of the house has 4 zones of thermostatically controlled hot water heat.
As far as just insulating a garage shop door - that will help, but that skinny rubber wipe on the edges and the bottom seal (chipmunk food on my garage door frames) are really a big 0-R value air gap in the cold air off an iceberg windy winter and unless there is something more substantial around those edges.
Quando omni flunkus, moritati (When all else fails, play dead - R Green)