What rootstock has it been grafted onto?.
Fruit trees are normally taken as cuttings and then grafted onto a specially grown root. It is this root that determines how big it will grow.
Hopefully you told someone you wanted to grow it in a container and they sold you one with a dwarf rootstock like M27 or M9.
There's a little article here about apple trees in containers.
http://minervasgarden.com/Apple%20Trees%20in%2 0Containers.htm
Try and find the rootstock on the label and look it up on the internet. If it's going to grow big, give the nursery a ring and tell them that you wanted to grow it in a container and see if they'll exchange it for a dwarfing one.