I think this is a bad idea. Avocados require freely draining soil, and if you're planting inside an IBC, then you're going to need most of a cubic ton of an artificial soil mix. But most of that might be wasted since the tree is shallow rooted and the main feeder roots are in the first 6 inches of soil. Trees can potentially grow 60 feet high, so you're continually going to have to prune it to keep it under 2 metres, but it's already one metre off the ground making your task harder.
When planting a dwarf avocado they suggest you start with a 60 cm pot, and repot every second year, and change the soil.
Your IBC might last 5 years more or less under NZ's high UV levels just when your tree is about to start first fruiting. And then you'll need to repot it.
If you're looking for a meaningful amount of fruit and you have to plant into containers, then maybe purchasing a number of dwarf varieties is the way to go. You'll do better with two avocado trees anyway for cross pollination, a type A and a type B.
Dwarfing is usually done by grafting the scion onto a dwarf root stock. The root stock controls the height of the tree.