It is true that doggie bags are not part of French culture. That said, takeout is common. Many people buy baguette sandwiches, Chinese food by the kilogram, and Greek gyros to go. If an eatery offers takeout, then it follows that they have ways to wrap up your meal for the commute.
Even at these places, however, you may ask for a container of ketchup to go with your fries and receive this.
I appreciate that they didn’t blink an eye when I asked. I didn’t either when I received ketchup in a ball of foil.