Growing onions in containers is very easy. The roots really do not need deep soil but they do need access to plenty of nutrients to help them grow well. I used 18 gallon containers with organic compost created from my kitchen garden. This mostly consisted of leaves and clippings from the kitchen.
The onions grow well with a reasonable amount of daily watering. Each time I added water to these containers it was in the morning for over 80% of the time which seemed to work well. The containers that did not receive as much water dried up or did not grow healthy green shoots. These onions were grown in Los Angeles California and one of them will be eaten today with a variety of mushrooms, garlic, ginger, and other helpful nutrients. Enjoy!