I suggest addressing the leaving for a better job issue head on. One possible tactic is something like "Yes, I probably won't be around forever. But as long as

I suggest addressing the leaving for a better job issue head on. One possible tactic is something like "Yes, I probably won't be around forever. But as long as I'm working for you, I'll show up when I'm scheduled to work, on time, sober, and ready to go. I have a car in good conditions / live a ten minute bike ride away / am on the X bus line. When I leave, I'll give you two weeks notice so you have time to pull me off the schedule."