Buildkit: build-using-dockerfile example shouldn't create temporary files

[tonistiigi]

When building big images this can cause disk space issues as the temp path can't be controlled. Also, the error isn't that helpful.

Instead, we should pipe directly to docker load. buildctl --exporter=docker can already pipe directly.

@vdemeester @AkihiroSuda

[AkihiroSuda]

yes, working on this now

[AkihiroSuda]

https://github.com/moby/buildkit/pull/326