Hm that might be tricky. Is there a triangle with a number on the bottom like 1, 2, 7 etc? Some plastics might be a bit permeable. If I were really set on using this soap bucket for brewing I guess I'd probably rinse it thoroughly with warm but not hot water. Then I'd fill it to the brim with cool water and let it sit at least overnight or ideally for a couple days. Then I would taste that water in the bucket. If it has any off taste like soapy or perfumey then it's not ready yet. So maybe try another water change let it sit again and see. If it doesn't clear up by then I'd say don't use it for brewing, because that taste is definitely going to get into your brew as it sits in there for a few weeks, plus the alcohol might pull out even more than the water.