Buildkit: ARG in COPY --from: invalid reference format

[folex]

Dockerfile:

# syntax=docker/dockerfile:experimental

FROM scratch as production
RUN echo production && touch /artifact

FROM scratch as test
RUN echo test && touch /artifact

FROM scratch
ARG environment=production
COPY --from=${environment} /artifact /artifact

How I run it:

DOCKER_BUILDKIT=1 docker build -f tools/docker/Test.Dockerfile .

Output:

[+] Building 1.5s (4/4) FINISHED
 => [internal] load build definition from Test.Dockerfile                                                                                                                                              0.0s
 => => transferring dockerfile: 299B                                                                                                                                                                   0.0s
 => [internal] load .dockerignore                                                                                                                                                                      0.0s
 => => transferring context: 2B                                                                                                                                                                        0.0s
 => resolve image config for docker.io/docker/dockerfile:experimental                                                                                                                                  0.7s
 => CACHED docker-image://docker.io/docker/dockerfile:experimental@sha256:888f21826273409b5ef5ff9ceb90c64a8f8ec7760da30d1ffbe6c3e2d323a7bd                                                             0.0s
failed to solve with frontend dockerfile.v0: failed to solve with frontend gateway.v0: rpc error: code = Unknown desc = invalid reference format

What I'm actually trying to achieve is to build files for production, but copy them from filesystem for tests. The example I provided is a simplification of that :)

P.S. I use experimental syntax here since I actually need --mount=type=cache in actual build, but I get almost the same error without it

[tonistiigi]

you can use:

ARG environment=production

FROM scratch as production
RUN echo production && touch /artifact

FROM scratch as test
RUN echo test && touch /artifact

FROM $environment AS currentenv

FROM scratch
COPY --from=currentenv /artifact /artifact

This is explained in https://medium.com/@tonistiigi/advanced-multi-stage-build-patterns-6f741b852fae

[folex]

Oh, I missed that in the article. Thank you so much! The article is just awesome.