What should I use to clean out a plastic bucket I am planning on using to store food in?

If you are worried about the virus, then soap or dishwashing soap is better rather than using bleach for washing plastic buckets, the smell from bleach would stick your plastic bucket.

If you aren't convinced it is safe using only soap, you can try to disinfect it with alcohol, just spray it or swap it.

What, if anything, was in the bucket before?

I prefer to stick with food grade buckets, most easily found from foodservice operations that get food in buckets.

For those, hot water and baking soda are key for removing the smell of the previous food if it was smelly, after washing with soap for basic cleaning. i.e. olive kegs are handy, but I can wash them by hand repeatedly, in the dishwasher repeatedly, and still tell that they were olive kegs by sticking my nose in and sniffing. But a few iterations of hot water and baking soda (sodium bicarbonate) left to soak and they don't smell like anything any more. Also works for peppers, pickles, molasses and other odors that soak into plastic.

A hardware store new empty bucket that wasn't used for anything, but which is not marked as food grade (such as the "fork and wineglass" symbol being molded into it) is probably OK for storing rice, but I would not use one for wet food storage (pickles, etc.) and would prefer a food-grade bucket even for rice, myself.

A bucket that had non-food chemicals stored in it I would not use for food storage at all, no matter how well cleaned out it was, barring some sort of post-apocalyptic scenario where I could either starve or use a cleaned out paint or grease or detergent bucket to haul food in.

Another option is to put the food in food-safe bags, and put the bags in the (non-food-safe) bucket for ease of moving and protection from damage, rodents, etc. The food only touches the bag, not the bucket.

There’s a great cleaner and disinfectant that kills 99.9 percent of Bacteria and fungus and it’s called spray nine. If you’re concerned about the coronavirus any cleaner that you have that will do the right job will be having a DEP number on it I do believe and that’s because of the chemicals used to disinfect. Any cleaner that has the two sets of government regulated numbers on the bottle is more than ready to kill any bacteria or virus