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If the tree is small then it will be fine for a few years. It would be better to have the tree in the ground if you could though. Watering will be a demand especially when in leaf and fruit is forming.

As Eileen says the tree would be fine on a dwarfing rootstock for a while but if you were to forget to water at a critical time you will have problems. In the ground would be best. If you haven't made the purchase you need to consider that Bramley is a poor pollinator being a Triploid variety needing two other varieties that come into flower about the same time.