Cross-arch images are supported, but you can only virtualise the arch you’re on. No running x86 on ARM for example.

Edit; see replies, I got this wrong

This isn’t true, there is a --platform flag which can be used to emulate another arch: https://docs.docker.com/docker-for-mac/apple-m1/

It’s fairly straightforward to configure QEMU to do the emulation. And the Docker preview released a few days ago does that without even asking.