You can use hot soapy water with an ounce of bleach to fill the bucket, let stand full for 10 minutes, wash again with soapy water, rinse well several times.
The bucket should be a "Food Safe" bucket (NSF). If it is not rated "Food Safe" you can use a "Food Safe" bucket liner.
You can also use food safe 5 gallon bucket liners. I have several food grade buckets that are sanitized, and I put the liners in them keeping them clean as I use them.
Food Safe grade buckets are available at bakeries, delis, and such... Most are more than happy to give them to you.
http://starkboards.com//Disposable_Cutting_Boards_files/widget0_markup.html#!/5-Gallon-Bucket-Liners/p/28037235/category=8946173