It's fine for many fig trees to go below zero. Of course, figs planted in-ground are more cold tolerant than those in pots. For example, say your fig would stand up to temps of -15ºC planted in the ground, if you plant the same fig in a pot, it could possibly only handle -5ºC because the root system is closer exposed to cold thanks to the structure and above-ground nature of the pot.
Do you know the average low of temps your detached garage has?
How thick is the wall of the pot you are using?
Also, do you know what variety your fig tree is? Some varieties are more cold hardy than others.
PS - one piece of advice: It looks like you are using straight potting soil with no mulch. I would definitely mulch your pot with a good 1" layer at least of something like pine bark. I would also consider using a potting mixture instead of potting soil to help the soil drain better.