Image build is failed when the username has uppercase

[johnguo1019]

https://github.com/rayonlabs/chutes-api/blob/c64e5217b4257c8f407dece917ffd22dfd313d86/api/image/router.py#L173

When users try to build images, it gives a following error when the username has uppercase.
I think the issue can be fixed by updating username to username.lower()