It's absolutely irrelevant since Docker (cgroups) are just the Linux kernel abstraction which helps to isolate resources of the processes. And with this image the general idea is that it comes like a package, with "just take & run" approach, eliminating the need to depend on the specific Debian-based Linux distro (which is required by Steam and provided with this Docker image).

Then as discussed in this thread, it gives few security advantages, control benefits since those isolated resources are controllable.