I'm going to try a soaker hose. I plan to have the hose coming down to the first container (water should follow the hose down), make a loop around the edges and exit up to the next container. There I will attach it to something and let the hose trail down to the next and loop it again. The little water that drips between the containers will be either collected or land on some material that will end up wetting my compost pile.

That's plan A anyway.