Rain barrel cleaning question

So I recently got a plastic barrel for a rain barrel but I wasn't paying attention and got one that had previously had fabric softener.  There is still a good bit of residue still in the barrel.  Is there any good way to safely remove this residue from the barrel, i.e., using some sort of solvent? Or is this barrel a lost cause for this purpose?  Thanks!

Can you wipe it on clean rags and use it in the wash?

Unfortunately I can't reach into it. The two openings at the top are probably only 2 inches in diameter each.

Try a hose with a jet nozzle