Is Docker still using a VirtualBox based solution? I thought they were using the macOS virt framework.

No, it has never used virtual box. Originally it used hypervisor.framework. I wouldn't be surprised if they now use the virtualization framework (newer higher-level abstraction on macOS).

I doubt they're using VirtualBox. VirtualBox didn't even run on Apple Silicon until 9 months ago, but Docker Desktop still worked, so it must have been using something else.

colima allows to chose between qemu and apple virtualization framework.

I found there is not much diffence in performance between both on my intel macbook.

I think it will not take long until rancher also updates to latest lima to support apple virtualization. but I haven't checked the latest release notes.

That's because QEMU can do hardware acceleration in an x86 host but not in M architecture. But as far as I know, only the Apple framework can do hardware acceleration or use Rosetta 2 to make things near native speed on an M-series Mac.