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There can be many reasons for this to occur. Can you include a photo, and a little about how the weather and rainfall has been?

This will help in the meantime: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-fruit-stays-small.htm

I would recommend switching to a dry, slow release, complete organic fertilizer like the excellent products from Down To Earth https://www.downtoearthfertilizer.com/products/blended_fertilizer/vegetable-garden/

or the best in my opinion: http://soilminerals.com/Agricolas4-8-4_MainPage.htm

If applied during the growing season, top dress and cover with mulch. And work it into the soil before next planting.

Tomato hornworm is the caterpillar of one of the Five-spotted hawk moth. The only way to prevent them is to deny access to the plant by the moth. The adult is 3-5 inches long so cover the plants with netting small enough to keep them out. Examine plants daily, especially if you see adults moths in your flowers at dusk. The eggs are like a pearl and white to light green. They are 1-2mm (large for an insect egg) and are deposited one at a time, often on the upper leaf surface. Also look for hornworm "frass" a term that just means excrement. If small black round clusters show up on your plants, you have a hornworm hiding somewhere! More info here: http://entnemdept.ufl.edu/creatures/field/hornworm.htm

This looks more like blossom end rot although you are correct when saying that hornworms feed on tomatoes and not just leaves. If you haven't had a hornworm infestation the blossom end rot is likely. It is very common and often related to irregular moisture levels- going from very dry to a heavy watering, for instance. In some situations, low soil calcium, high pH and high phosphorus levels interfer with calcium absorption. Your photo looks like the area has been sealed off by the fruit. You can still enjoy this tomato; simply cut out the affected area. https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm

This is powdery mildew. This article will help you to correct this: https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm

I have my garden in container, exclusively, each year. There are several things to know before you can get a good harvest.

You need drainage. Make sure all containers have drain holes. After about 2 or 3 months, the soil will be depleted of all fertilizers. You will need to replenish them. The fertilizers that you use should not contain slow release nitrogen, such as miraclegro. This will drastically reduce fruit number and size, while giving you amazing leaf growth. Make sure to keep dolomitic lime around, and give all plants a handful every few months. This will supplement calcium, and magnesium. These are the first nutrients to be depleted, and these will also keep your soil pH stable.

These, alone, should solve almost all of your issues. There is still a lot to learn about having a container garden. I do it every year, and have been for a long time, and I still learn new things each time.

This collection of articles will help: https://www.gardeningknowhow.com/special/containers