How much of root space apple tree on B9 needs?

I planted Ginger Gold on B9 into a spot now I have doubt about. It is edge of the lawn, with 5’ stone wall drop on north-east side. I planted it 2.5 foot from the drop hoping the roots will go south- south west. Today I was going to install drainage pipe around the tree (the lawn has a slop and in big rains water can just run down to the planting side) and found that there is a huge bolder on the south side about 2.5 foot from the tree trunk. Basically, tree is planted in 5’ diameter in-ground container about 3’ deep (there may be a bolder bellow that deepness). I do not worry much about drainage, as the retaining wall has several water outputs and I am going to protect the planting side from water that is running down the lawn with drainage pipe. But will be 5’ “container” enough for tree to grow? The tree is staked.

On B9 that is more than enough.