[builder] add docker buildx bake as a builder

First off, I wanted to say thank you for putting together this tool. The ability to specify local build context and have they transformed into images is great!

I have been using [Docker buildx bake](https://github.com/docker/buildx#buildx-bake-options-target) as a similar tool, using the Kubernetes executor to build images on k8 and push images.

I have been appreciating the ability to parallelize the build DAG as well as add caching for certain steps easily. See [my generated docker bake json](https://github.com/data-apis/python-record-api/blob/4fa06df6c0eac85e9f54d70f7172dde9e1ef3125/k8/Makefile#L123-L169), for an example of the caching and tags.

So I am curious about two points:

1. Are you using [buildkit](https://github.com/moby/buildkit/) to parallelize the builds and to support caching?
2. Have you considered using the same JSON syntax as Docker Bake or working with them to come up with a common form? It seems like the tools are trying to specify very similar features. Or could you just use that tool directly?

These are larger discussion points, so I more just wanted to get a sense from you all how you are thinking about the relationship between the two tools.

I am very thankful that there is so much great work going on this space right now!