I have never had experience with growing grains for actual grain. However, i do grow winter rye as a cover crop in my garden and i know lots of people who grow wheat in containers but for the juice of the leaves, not the grain. In reality though, you will probably only get a trivial amount of grain from plants in containers, if you get any grains at all.. Remember that the bigger the container, the bigger the plant, generally.

Personally, I wouldn't even waste my time planting grain in my garden, let alone in a container.

I have started wheat, oats, barley, rye in trays and small pots and transplanted them to my home garden outdoors. I haven't tried to let them produce grain while growing in the container. I guess it would be easier with oats since they are a smaller plant. I guess the best is to grow the cereals in a box that is 1 foot deep and about 2 x 1 foot wide.

Beware that cereals like to experience a chilly period followed by a warmer and dry period, with plenty of sunshine.

I have sucessfully grown corn/maize and millets in rather small containers (1 foot deep). The corn were about 6 plants and then I had to hand-cross-pollinate them

Remember that the cereals like wheat also need cross-pollination by wind or your hand, if you expect grain forming. And it will take a few months from seed to ripen grain. Oats are faster, wheat takes longer.

Assuming proper water and nutrients, the size of the containers will determine if you get any grain and how much IF grown outside.

But trying to do it indoors is starting out with at least 3 strikes against you. It is just too much of an artificial environment for the plants to expect any success. You simply cannot compensate for the sun exposure required plus all the other benefits of growing out doors. Sorry.

Dave