I've never had much luck growing tomatoes in containers but perhaps others have, cherry tomatoes do ok though. I assume that container has drainage holes in the bottom? If not a soggy bottom issue then Early Blight would be my best guess but it's just a guess.

Yep, it has drainage.

Have you been supplimenting the miracle grow mix with some added fertilizer? Yellow leaves can sometimes indicate low nutrients, esp nitrogen.

I've never used fertilizer on any of my plants and others look perfectly okay.

PART 1 - Why your plant looks sickly.

Tomato plants rank priority for available nutrients to the fruit, followed by growth and then roots. When phosphorous, potassium, nitrogen and magnesium are unavailable from the roots the plant takes these nutrients from the bottom leaves to supply the fruits to produce seed, seed is it's main objective, and will sacrifice foliage to achieve it's goal. The first sign of low or unavailable nutrients is if the plant stall out. There should be at least 6 inches of new growth per week even when fruit is hanging on the plant for an indeterminate tomato plant, and what you have. Miracle grow potting soil has little fertilizer in it, and alone will never grow a tomato plant through even half a season. They needs 14 nutrients they aren't getting, and no different than a human not getting all the nutrients and minerals they need.

Part 2 - The fix.

The plant needs fertilized. Organic fertilizers for a potted tomato plant will usually be ineffective because of the long time lag of weeks to month from application to become available to the plant. The fix is synthetic fertilizers plants can absorb immediately, not next week, or next month like organic, but the day you apply it.

and this is where it gets complicated, finding an all-in-one fertilizer that has all the nutrients in it your plant MUST have now is nearly impossible . They just don't make one with everything a plant needs because of chemical incompatibility, meaning how certain elements react, lock up, or fall out when mixed together. So at least two different fertilizers must be used.

Part 3 - finding good fertilizers.

there isn't one for tomatoes, and why I mix my own from many sources. The few that do exist are questionable, cheating Peter to pay Paul and trying to push the boundaries of chemistry that aren't formulated for Tomatoes.

Part 4 - Crossing industry lines.

One solution is turning to products catering to Cannabis growers in pots for ease. Tomatoes and cannabis are nearly identical in their heavy-feeding habits, and If you can master the exact nutritional needs of a tomato plant, you can grow cannabis flawlessly, and vice versa. Honestly the market for premium cannabis fertilizers is much better than tomato fertilizers and why these fertilizers exist on the market and not good tomato fertilizers.

If you'd like to expands on any of these parts let me know, I'd be happy to explain in more detail, or direct you to the products you would actually need to grow tomatoes in pots like a pro.

Thanks for the detailed info. I don't think I'll try container vegetable gardening (I know peppers and tomatoes ate botanically fruits) next year. I spend more, have more problems and get less in return than just buying at the grocery. It was a horticultural experiment to start, anyway. My peppers did well until a squirrel or chipmunk helped themselves.