Not corn snake related but I need help figuring out what snake this is!!!!!

So I live in western Montana and while walking around my apartment i found a snake near the heater. Yay for me! I caught it with a bowl and I don't want to let it go since its so cold out here I don't think it would survive. It's super small and very aggressive(although that's very understandable) The main native snake here is a rattle snake but there is no rattle or anything resembling one on its tail. I have an extra 10 gallon my corn outgrew, should i keep him for the winter or release him into the snow. What should I do? Any ideas to what kind of snake this is? sorry the pictures are bad. Any help is appreciated!!!!