Add a per-build flag to limit concurrency / parallelism

Description

Currently the only way to limit parallel builds is via config, added in https://github.com/moby/buildkit/pull/2049. It would be useful to be able to tune this per-build, for cases where parallel RUN commands are problematic, but only in that specific build and not system wide.

For example, I have a system where normally the default behavior is fine. However, if one stage is doing a very large download, it might choke downloads in other stages due to network congestion. This is a problem unique to the multi-stage Dockerfile that is trying to download a large model in one stage, and packages in another stage. I do not wish to retune my entire Docker daemon for this issue, so instead I'm simply disabling buildkit with DOCKER_BUILDKIT=0 docker build

This, or something very similar, has been hinted at but as far as I can tell not formally requested:

• https://github.com/moby/buildkit/pull/2049#issuecomment-809924583
• https://github.com/moby/buildkit/issues/1032#issuecomment-938252355

The current solution from https://github.com/moby/buildkit/pull/2049 does not meet this need, as that implementation is daemon-wide. The follow-up proposal in https://github.com/moby/buildkit/issues/2108 does not meet this need as there are times when the user will know better than the automated system, for example when there is a bottleneck on the internet link not easily detected. Please add a flag to docker build to allow tuning this per-build. Thank you.

Start at the docker build flag/option path and the existing daemon-wide concurrency configuration described in the issue. Trace how a per-build limit would reach BuildKit, then verify that parallel RUN commands are limited for one build without changing daemon-wide behavior.