Steam provides a stable Linux runtime, but it's not containerized or isolated Docker/Flatpak-style. It's closer to a chrooted env with some specific distro, but without chroot and the need to maintain said distro. They want to provide runtime stability and compatibility comparable to that on Windows - it's a great initiative, and I really hope they'll succeed. The snowflake-like userlands on Linux are a pain, but the current solutions (Docker, Flatpak, things like conda) are all bad solutions to this particular problem (though they are good solutions to other problems, so it's not a criticism, just a difference in goals).

However, Steam Runtime for Linux was still in beta, last I checked. Plus, it doesn't solve the cross-platform part. But for Linux-native development, Steam Runtime might be what's needed to have long-term compatibility for apps (finally).

It's xkcd927 of a container.