Don't bud -- I was told that Gardenias need a 10 degree difference between day and night temperature to set buds.

Buds fall off -- I was told that the buds abort when they don't get enough moisture (water daily -- but not wet feet).

Leaves discolor -- I was told that the discoloration is due to missing nutrients, use a fertilizer with a high first number.

These are the things that I've been told. I am trying gardenias in a container for the first time this year too.

I have been successful caring for my container gardenia. I try to meet all nixonk's points for gardenia. I would also add (lessons I learned from last growing season):

1) Container gardenias need some morning sun (NE) daily.
2) They need fertile, moist, and acidic potting soil:

*Keep the container soil moist.
*Before adding iron chelate, add coffee ground regularly first (increase acidity). Pile on layers of coffee ground was sufficient to keep the leaves from turning yellow and reverse the yellowing, and it kept the leaves green with the need to add chelated iron.
*Fertilize regularly (1/4-1/2 of recommended acidic fertilizer or chemical fertilizer at each watering) except when blooming.

Yellowing and defoliation (2-3 weeks time?) are sometimes necessary at the start of new growing season/cycle/growing condition. In such circumstances, no soil ammendment or special attention is needed.

In the fall and/or winter, container gardenia can use some cold temperature and could stay outdoors as long as you can keep it above 25 degrees F. I keep the container gardenias indoors for a few weeks/months in the winter when the temperature drops below 25 degrees F, and then slowly acclimate them to stay outdoors in the Spring.

My container gardenias are Veitchii.

Quote: "Pile on layers of coffee ground was sufficient to keep the leaves from turning yellow and reverse the yellowing, and it kept the leaves green with the need to add chelated iron."

EDIT: It should read, "Coffee grounds kept the leaves green without the need to add iron chelate."

The minerals and fertilizers I used:
I added Epsom salt (no liming).
Slow-release fertilizer with minors.
Use acid-based fertilizer.
When I used high phosphate chemical fertilizers (high middle number), Miracle Grow Tomato Plant Food, or MG Rose Food at the 1/4-1/2 of the recommended amount at each watering instead of an acid-based fertilizer, I still got healthy buds and blossoms. Without using an acidic fertililzer, I do pile on coffee grounds regularly to keep chlorosis away.
I conditioned the soil with some compost.

If your container gardenias will be grown under hot sun all day without filtered shade, you may have to water more often daily to keep the soil slightly moist (but not water logged).

The container potting medium should drain fast but kept moist. Any reputable potting soil straight from the bag should be fine for your expected hot sunny growing condition. Adding perlites for improved aeration may be optional and unnecessary if the container gardenias will be grown in a hot sunny location and if the plant/container is relative small (10-12" diam). You'll have to experiment a bit the first year (one grown with perlites and one grown without). Topping compost and coffee grounds should help with moisture retention as well.

I also amended the potting soil with super phosphate.

Two more buds just opened this morning after some overnight rain. Many more blossoms to come. Such pure white blossoms with waves of light fragrance on a cool cloudy misty morning. What a blessing!

Constant moisture above and below the plant (and temperature to some extend) is key to success in preventing foliage and bud drop, for Gardenias are tropical plants.

Potting gardenias in a slightly smaller container may promote healthier and more prolific growth.