You are probably remembering the important 'e' rule for feminine words. "Douchebag" would be somewhat of an exception to it; literal consideration is meaningless ("douche", which is feminine, simply means "shower") and the foreign word latched on to the end would not register. Morphologically speaking, I am certain it would be rendered feminine.
As far as it matters though, you would be correct. Bag (sac - in French) is masculine and would be the subject that decided the form when translated.
I do not actually know if "douchebag" even has a simpler equivalent in French. I am intrigued enough to do some research into it. I shall report on my findings.
Thank you.