I know a lemon tree has shallow roots and mine is obviously sensitive to things like not enough water, not enough food etc. I once planted pineapple mint in the container with my lemon tree. It got root bound by going round and round in the top of the container. I was sure it was the reason for our tree's poor health and lack of fruit. Since I took it out the tree has actually fruited, although not nearly as much as I want (only one this season). This could be because it is a rough-skin lemon, though, and all the pictures I see are of eureka and meyer lemons.
The container is round, about 55cm in diameter and 45cm height, so about 427 litres or 112 gallons.
It has never really been truly happy but it has improved. I put a parsley in there last year but I just took it out. Should I consider anything else, perhaps something complementary or something that won't hurt it? Or should I simply leave it bare? Which I don't like actually.