OP isn't talking about virtual environment style sandboxing, they're talking about sandboxes that prevent arbitrary code from deleting or stealing any information your user account has access to on your computer.
The solution, therefor, as always been to use the OS tooling for that. Even the .Net ecosystem eventually went into that direction.
The JS ecosystem is making that mistake right now, and will of course, deprecate this API in 10 years after they realize they can't make it secure either unless they basically reimplement BSD jails entirely.
Firecracker and gVisor provide much stronger isolation. Both are battle tested; clouds run millions of multi-tenant workloads on these every day. Docker would simply never even be a candidate for this purpose.
If you have a working docker escape exploit at hand, that works on unprivileged containers, you can earn some good money. Just saying.
Docker was not created as a sandbox, but people rely on it for security and it is a sandbox at this point. Hell, containerd is one of kuberbetes backends and it absolutely relies on it being a secure sandbox.