I'm surprised no one here has mentioned the d2x-rebirth source port here, a fan made native port that runs great on Linux. Even if you're not a Linux user, you probably want to play this game at a resolution higher than 640x400. Some distros allow you to get it from the package manager, and on Arch and Manjaro you can just use the AUR. (note that you will either have to install the game data for d2x-rebirth to detect it, or use can just use command-line options to specify the Steam installation.)

I never bothered with this, but if you however want to play the original DOS engine of the game, there are better ways to do so than using Proton; it really is a waste when DOSBox itself is native to Linux. Someone below mentioned something called boxtron, but you could probably also use vanilla DOSBox without any frontends, and I think I read somewhere there's a DOSBox flavor that supports 3dfx to OpenGL translation if you want the game to actually use your GPU.

alt+tab does not work in fullscreen, but alt+enter to switch between windowed and fullscreen does.