Optionally use buildx to build for multiple platforms

[manics]

• New argument --builder to switch between docker build and docker buildx. I've used a choice instead of a flag in case we want to add more builders in future.
• New argument --platform that takes platforms accepted by docker buildx --platform

[consideRatio]

The suggested API makes sense to me!

[consideRatio]

Nice this looks good overall so far in my mind, i left some comments about thoughts that popped up.

[manics]

I've switched to an Enum which I think is a lot clearer! Also added some tests.

I think this is ready to review now. I'll add an option to skip an image (for the Z2JH singleuser image) in a separate PR.

[consideRatio]

This looks good to me except that I didn't understand a part about providing * in a list of parameters for a few functions.

I guess there will be a sensible explanation for that though, and then this LGTM!

Thank you @manics for your work! ❤️