I don"t think it would hurt the dryer a bit to vent it to the interior of your garage. It might be an easy and fairly cheap way to get some heat when you occasionally need it.
On the other hand, do you really want to vent the moisture to your garage, especially if it is pretty close to airtight? Moisture inside can cause all kinds of problems to buildings and what is stored inside them.
I think I would vent the dryer outside. Good luck!