Docker is a lot more than just an unprivileged user. In particular, it comes with a seccomp filter. A lot of LPEs are blocked by that filter. Docker is actually a quite decent security boundary - in this case the attackers did not attempt to subvert docker by attacking the kernel attack surface, they attacked a weak configuration via the docker socket being mounted.
The reason you don't use a "grown up VM" is because it's significantly more difficult. Which VM? Firecracker requires KVM and a guest operating system - so how are you getting things in and out in a way that doesn't violate security? That's real work.
gVisor is great and my recommendation, certainly, but the difference between "nothing" and "docker" is actually pretty huge imo.
The reason you don't use a "grown up VM" is because it's significantly more difficult. Which VM? Firecracker requires KVM and a guest operating system - so how are you getting things in and out in a way that doesn't violate security? That's real work.
gVisor is great and my recommendation, certainly, but the difference between "nothing" and "docker" is actually pretty huge imo.