Add ability to pick builder version

[pjar]

I'm on M2 max, and after updating docker desktop builds for amd64 builder started to throw a seg-fault. It's known a issue in buildx https://github.com/docker/buildx/issues/2028 but yet to be fixed. One workaround would be to switch to amd64/v2 builder as suggested in the comments there. Kamal hardcodes builders so in such cases it would help to be able to change builders.

[djmb]

@pjar - could you share your builder config?