That sounds like inhumane treatment to me. Oh, you meant putting the
tree in a pot. I get it.
How about using a stake instead? I mean for the tree? Or maybe a steak, for the dog?
If you do need to put it in a pot for training purposes, bigger is better for a fast-growing tree like a trident. Maybe one of those medium size (10 gallon?) plastic 'faux terra-cotta' patio pots to get it up out the dog's line of sight and targeting. With a big pot like that, you can use potting soil, maybe amended a little with orchid bark and/or crushed lava.
Regards,
Matt