Proposal: add --platform option for COPY --from=(image|context)

[dsseng]

Docs for the feature to which an improvement is proposed: https://docs.docker.com/reference/dockerfile/#copy---from

Currently, if an image or context, containing an image, is supplied into a COPY statement, the platform chosen is either the target platform of the image being built or the only platform that image has.

However, if this image contains for example cross-compilation-relevant assets, it might be desirable to be able to select its platform. For example, if I need to add Go image in this way, I'd pass COPY --from=golang:1.23 --platform=$BUILDPLATFORM / /gotoolchain to obtain a toolchain running natively, without emulation, on the build host.

This argument is expected to work in the same way as --platform in https://docs.docker.com/reference/dockerfile/#from

Any comments are welcome!

[tonistiigi]

FROM --platform=linux/amd64 img AS assets_for_amd64

FROM ...
COPY --from=assets_for_amd64 . .

[dsseng]

AFAIK FROM doesn't support contexts (I mean oci-layout or docker-image:// passed in buildx context)

[tonistiigi]

It does

[dsseng]

Thank you! This proposal would still give shorter build instructions, so I would prefer if it's left open for others to comment on

[tonistiigi]

I don't think we want this though. --from is supposed to be a selector based on the name without any differences based on the internals of what this name means. Stages are how you control if you want a special behavior for a name (same with variables for example).