I don't know much about Docker, but I know Steam for Linux is built for Ubuntu. Maybe Docker helps for distros like Fedora? The benefit is probably highest in the case of NixOS, because Steam changes stuff behind the scenes and so isn't really compatible with declarative package managers.
NixOS supports Steam wonderfully - it creates a half-container (alternate filesystem root, shared networking) which looks like what Steam expects, has full access to graphics drivers and your home directory, and can be executed like any other app (run "steam"). The full Docker infrastructure isn't really necessary.
@tormeh look at a Docker just like at a system that gets you the images (in "tar" archives) from the Docker Hub repo (or build them by yourself) and lets you run these "tar" images by leveraging the cgroups (Linux kernel abstraction feature that limits, isolates resource usage of a process). You may basically look at a Docker like at "chroot-on-steroids".
And yes, it helps to run the images wherever you have Docker installed. There are few limitations though, especially when it comes to the point one needs to pass something from the host into a container (e.g. driver libraries, devices, special system paths or environment variables), since they may differ from distro to distro, from Platform to Platform.
NixOS generates a FHS compliant chroot for steam to be installed to, so it can do whatever it wants in that playground.