Cherry tree leaves curling

I have a 6' young cherry tree in a 18" pot - it is officially 'designed' for pot life, though I'm not sure it got the memo as it doesn't look too impressed with pot life... I water it regularly (3 times a week at least) but the leaves, whilst perfectly healthy looking curl inwards. There's no sign of disease or aphids... well, that's not true, the very end leaves were covered in aphids but I was advised to nip the last couple of inches off, which I did a couple of months ago and that solved that issue.

Is it lack of water causing the curl? It did produce 4 (!) very nice cherries.