bogus warning on bake --call=outline

[tonistiigi]

Calling docker buildx bake binaries --call=outline on a mac return among other things

 => WARN: Requested platform "darwin/arm64" does not match result platform "linux/arm64"                                                                   0.0s

 1 warning found (use docker --debug to expand):
 - Requested platform "darwin/arm64" does not match result platform "linux/arm64"
binaries

This seems to be some conflict between this target requesting a mac binary as a result and no actual result being created because of the --call=outline.

[jsternberg]

What should happen if --call=outline is called with multiple platforms? Should we return an error? The outline structure doesn't really seem to be multi-platform friendly.

[tonistiigi]

I think combining wouldn't be that bad. Different platforms can use separate build-args but atm same arg can't have multiple values. If different values per platform will be supported then I guess platform should be part of arg key if there are multiple.