Restore build concurrency limit now that bake supports it #14313

I'd love to see --parallel / COMPOSE_PARALLEL_LIMIT un-deprecated and reused, now that Bake has per-target limit in buildx v0.38.0 (yesterday!) docker/buildx#4044

Since builds moved to Bake the flag is ignored (#13043), and was the main way to throttle builds so a low vcpu/mem CI server doesn't OOM in the middle of building a bunch of services at once. There's a workaround by writing a custom builder .toml, but that UX is not it, and I think that caps build steps not targets plus you have to remember to switch builders back.

I think Compose passthrough is pretty straightforward, but may need consideration around linked targets when additional_contexts: service:x become target: link so the build doesn't fail, and a flag when buildx isn't new enough.