First check with your program to see if this is allowed, some programs frown upon doing this and there can be consequences. Here's the deal, if you moonlight now you will make "decent" money but compared to attending salaries you are still getting the shaft. And during residency you are working many hours already. The time off should be spent with friends, family, or just plain old relaxing in my opinion. Otherwise burning out may creep up on you fast. It may be wiser to pick up a few more shifts if possible after residency since the number of hours will be less and the pay will be much much more.
Also remember residency you are still training, learning, and you want to have the best environment to do so. AKA, not being tired, burnt out, too stressed. And if you perform poorly during residency you can be kicked out, and then forget about that attending salary. Focus and learn as much as you can in residency, it will make your attending career much easier.