Feature request: push multiple stages

[LittleFox94]

Hi,

we have a Dockerfile with two stages, prod and dev, where dev builds on top of the prod stage and adds some more stuff for debugging, roughly like this:

FROM python:3.10-slim AS prod

RUN apt-get update && apt-get install ... && apt-get clean && rm -rf /var/lib/apt/lists/*

RUN useradd [..] app
USER app

COPY app/ /app
ENTRYPOINT ["/app/entrypoint"]

FROM prod AS dev
USER root
RUN apt-get update && apt-get install jq curl && [...]

USER app

It would be really great to have them built in one go, instead of rebuilding prod, pull it from the registry or pulling it from the cache. Something like this could work, = isn't allowed in the destination name, so this also shouldn't break any existing usage:

executor --destination prod=$our_registry/foo/bar --destination dev=$another_registry/foo/bar

Thanks for considering :)

[LittleFox94]

This could also be limited to stages that are already in the build workflow for the selected --target, so we'd have to add --target dev but before it executes the FROM prod AS dev, it would push the current state to the prod destination