`FROM --platform=$BUILDPLATFORM a as b` not overriding `TARGETPLATFORM` for multi-stage builds about buildkit HOT 7 OPEN

Yes, this is same as #4237 . --platform is a hint for pulling the image if it is a manifest list. In your case it is a stage that is already resolved to a specific platform.

Implicitly your code is:

FROM --platform=$TARGETPLATFORM busybox as base
ARG TARGETPLATFORM
RUN echo $TARGETPLATFORM > /plt
FROM --platform=$TARGETPLATFORM base as base-target
FROM --platform=$BUILDPLATFORM base as base-build

Additionally --platform=$BUILDPLATFORM does not change the meaning of TARGETPLATFORM inside the stage.

FROM --platform=$BUILDPLATFORM alpine
ARG TARGETPLATFORM BUILDPLATFORM
RUN env && echo "These do not match in cross compiling" 

from buildkit.

Additionally --platform=$BUILDPLATFORM does not change the meaning of TARGETPLATFORM inside the stage.

So basically FROM --platform=$BUILDPLATFORM doesn't do anything for intermediate stages? I guess I agree with AkihiroSuda from the other thread that that is quite surprising and a warning would be nice. Also the docs are very lightweight on this topic.

Do you think there's any other way to achieve what I want (besides duplication)?

from buildkit.

doesn't do anything for intermediate stages?

It only has an effect atm if the source parameter for FROM is multi-platform image or multi-platform build result(via named context). The stage itself can be any stage in the Dockerfile.

Do you think there's any other way to achieve what I want (besides duplication)?

If the commands are same for the stages defined with different platform sources(that result in same platforms based on config) then buildkit will internally merge the commands and they only run once.

from buildkit.