`FROM --platform=$BUILDPLATFORM a as b` not overriding `TARGETPLATFORM` for multi-stage builds

I'm currently investing in making our builds cross-platorm compatible. I want to e.g. download the uv package manager and use it in both our build and target images. I then hit the following issue, when trying to keep our build instructions DRY:

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

I would expect this to build two different variants of the base stage when BUILD!=TARGET, one for each. Like this works for multi platform images pulled from the registry. But this is not the case:

#On linux/amd64:
$ docker build --platform=linux/arm64 -f test.Dockerfile --target=base-build .
...
 => CACHED [base 2/2] RUN echo linux/arm64 > /plt
...
$ docker build --platform=linux/arm64 -f test.Dockerfile --target=base-target .
...
 => CACHED [base 2/2] RUN echo linux/arm64 > /plt
...

FROM --platform=$TARGETPLATFORM alpine as base-target
FROM --platform=$BUILDPLATFORM alpine as base-build

Here's the different behavior with images from the registry:

$ docker build --platform=linux/arm64 -f test.Dockerfile --target=base-build -t target .
$ docker run --rm -it target cat /etc/apk/arch
aarch64
$ docker build --platform=linux/arm64 -f test.Dockerfile --target=base-build -t build .
$ docker run --rm -it build cat /etc/apk/arch
x86_64

It seems like there's no other way to get this working besides duplicating the base stage for the different platforms and giving them a unique stage name. I'm happy to hear any other workarounds.

Possibly related to

• https://github.com/moby/buildkit/issues/4237

Start by reproducing the two builds from the issue with the provided test.Dockerfile, comparing the local base stage with the registry-image behavior. Review the related issue #4237 and BuildKit's multi-stage platform handling. Done means --platform=$BUILDPLATFORM and --platform=$TARGETPLATFORM produce the expected distinct variants without duplicating the base stage.