Allow ARGs in COPY --from and RUN --mount=from

[jedevc]

🛠️ Fixes #2717 (#2412 is also relevant)

Previously, using ARGs in COPY --from or RUN --mounts from was not supported, requiring using an extra intermediate stage. This patch allows global args to be used directly in these locations.

To do this, we need to resolve a dependency issue:

• We need to construct all the dispatchStates before evaluating.
• To construct the dispatchStates, we need to determine the fully-evaluated base LLB source.
• However, to fully-evaluate the base name using the Expand interface, we need to have constructed all the dispatchStates.

It is theoretically possible to refactor to not have this restriction, however, this is not required to implement the base functionality for global args.

To resolve this, we can use just the global args to evaluate the FROM fields using the toCommand helper, where the stage resolution occurs today.

At some point, it should be possible to implement local arg resolution as well here - however, this requires a change from the current approach of creating an intermediate dispatchState for those components.

@tonistiigi does this seem like a reasonable approach for a first pass?

[thaJeztah]

I think some of the hesitation at the time was that the UX could be confusing. For example;

ARG HELLO=world

FROM alpine
ENV HELLO=something
RUN echo $HELLO
COPY --from=$HELLO,...

In this, what value would be used for $HELLO? (in both cases)? (Probably same for cases where an ARG HELLO would be set in build-stage (FROM))

[tonistiigi]

This patch allows global args to be used directly in these locations.

I can see how this can be quite confusing that some mount values have different ARG values than others. If it is global, then we could just detect these cases and show a warning or error instead. Using the intermediate stage isn't hard, but it isn't obvious to a new user that they need to do it.

edit: looking at the code, I had forgotten that we already are covering this error case.