daemonless builds

[blampe]

Alternative builders like podman don't require a BuildKit daemon to be running but are still Docker-compatible frontends.

We currently have an exec: true mode which shells out to the docker-buildx binary directly. It would be nice to generalize this to additional tools, but

• What would that API look like?
• How would we map Docker inputs to the tool?

It would be nice to have something as general as possible, but it would be straightforward to define another exec mode which invokes a particular CLI and understands how to translate inputs into its expected format.

If you're interested in this feature, please give it a 👍 and let us know if you have ideas for how the API might look!