Clarify dabuild multi arch support
It looks to me that only docker on MacOS can support multi-arch as can be read here: https://docs.docker.com/docker-for-mac/multi-arch/
This makes it easy to run dabuild for a different arch with qemu emulation (its slow but it would work). I have tried this on Docker on Linux but it doesnt seem to work in the same way. I can also not find any reference for multi-arch for other platforms except the link above.