I would transplant the seedling to a larger pot, perhaps 6" across, and use a good soil mix such as Metromix. Growing plants in pots is much different than in the ground, and it's important to use a soil mix that drains well but also holds some moisture.

If the seedling grows enough over the summer, you could plant it this fall or transplant into a larger a container until it gets big enough to put in the ground. You don't want to move the tree inside during winter because it needs its dormancy, but you also need to protect it against temperatures colder than 15 degrees or so. You can do that by burying the pot in the ground or moving it into a space such as your garage where it will still get cold enough to go dormant but not cold enough to damage the roots. You can also insulate the pot.