Allow using variables in `COPY --from=`

[selurvedu]

Variables defined via ARG can't be used in COPY --from=$ARG /source /target.

Example:

FROM scratch
ARG SOURCE=debian:11
COPY --from=$SOURCE /etc/os-release /target

$ DOCKER_BUILDKIT=1 docker build
...
failed to solve with frontend dockerfile.v0: failed to create LLB definition: failed to parse stage name "$SOURCE": invalid reference format: repository name must be lowercase

FYI, the variable in a source or target definintion, e.g. COPY --from=debian:11 /etc/os-release /target/$SOURCE, gets expanded correctly both with and without DOCKER_BUILDKIT=1.

May be related to #815.

[tonistiigi]

Args in --from are currently limited to only args in global scope because using local variables would invalidate the dependency chain between stages. For how to use global args read https://medium.com/@tonistiigi/advanced-multi-stage-build-patterns-6f741b852fae