Just keep topping up the container with more compost and keep it watered. Many people do not realise just how much top growth there is on potatoes until they see them in containers.

Because you are growing several tubers in a confined space, they stretch with the competition and will get taller than those grown in the ground. I find the best idea is to stick 4 canes down the sides of the bags and encircle them with soft twine to support the stems, otherwise the stems will eventually flop over and break.