Here is a guide on how to clean such a bucket and make sure you can continue using it.
Vinegar. Add a cup full of vinegar inside the affected bucket. This will loosen the dirt from the stained surfaces.
Baking soda. Take three tablespoons of baking soda and add it to the vinegar. This will cause a reaction such that the gunk is completely off from the bottom. Let it sit for 15 minutes
Clean it. Put on a plastic glove and scrub the stains from the bucket using a scrub sponge. Rinse the bucket with some soapy water to clean it thoroughly.
Bid adieu to the stains on your bucket!