A couple of things can cause yellowing leaves like those in your photo - overwatering and nutrient deficiency. With this year's heavy rainfall it is difficult to decrease watering. The more water the plant in a container garden receives the more nutrients are washed from the soil. Yellow leaves will often be a sign of low nitrogen. You could increase your fertilizing to once/week or every other week. You should use a fertilizer formulated for vegetables as too much nitrogen can cause beautiful leaves and not as much fruit. With any water soluble fertilizer use 1/2 the amount of recommended fertilizer in your water. 

The yellowing leaves are a symptom and are likely not detrimental to the overall health of your tomato plant.