I have a Dockerfile where I would like to use the TARGETARCH that is set by buildx in my FROM line:

ARG TARGETARCH

FROM gcr.io/my-container-registry/my-container-${TARGETARCH}

...

(I'm aware that if I built my-container for multiple platforms in the same docker buildx build --platform x,y command, I would not have to use different tags for different platforms. But I don't want to do that since I usually want to build the images for each architecture on machines with that architecture.)

Expected:

Running docker buildx build --platform arm64 ... should result in FROM gcr.io/my-container-registry/my-container-arm64

What actually happens:

failed to parse stage name "gcr.io/my-container-registry/my-container-": invalid reference format

Using ARG TARGETARCH after the FROM line works as expected (although I did also run into #510, expecting it to work as @MarcelWaldvogel did).

Is there a way to get this to work? Or is this the intended behavior?