Multistage build: using builder image before it is built

[fragkakis]

Behaviour

I have a multistage docker build with 2 images: a builder image and a production image. The builder image contains additional things that I don't need in production. I am only using it to do some groundwork and copy the results to the production image. Pretty much the default scenario builder images are used for. My dockerfile is functional, it is being used in production for years. I am migrating its building to Github Actions.

I am seeing that the production image step that copies things from the builder image is being executed while the builder image is still being built. Notice below the first line vs the last two lines.

2023-08-10T18:45:02.0757808Z #16 [production  4/11] COPY --from=Builder /usr/local/bundle/ /usr/local/bundle/
2023-08-10T18:45:02.0758390Z #16 CACHED
2023-08-10T18:45:02.0759068Z 
2023-08-10T18:45:02.0759339Z #17 [production  5/11] WORKDIR /home/workable/app
2023-08-10T18:45:02.0759850Z #17 CACHED
2023-08-10T18:45:02.0760077Z 
2023-08-10T18:45:02.0763047Z #18 [builder 6/6] RUN gem install bundler:2.4.10 &&     bundle config set without 'development test integration' &&     bundle config set deployment 'true' &&     bundle config set path vendor/bundle &&     bundle install --jobs=4 &&     rm -rf /usr/local/bundle/cache/*.gem &&     rm -rf vendor/bundle/ruby/3.2.0/cache/*.gem &&     find vendor/bundle/ruby/3.2.0/gems/ -name "*.c" -delete &&     find vendor/bundle/ruby/3.2.0/gems/ -name "*.o" -delete &&     find vendor/bundle/ruby/3.2.0/gems/ -mindepth 2 -maxdepth 2 -type d ( -name spec -o -name test ) -exec rm -rf *** +
2023-08-10T18:45:02.0764396Z #18 CACHED

Expected behaviour

I would expect that the builder image is built first, and only then would the steps from the production image using it should appear.

Actual behaviour

I am seeing that the production image step that copies things from the builder image is being executed while the builder image is still being built.

Configuration

FROM gcr.io/staging-artifacts-... as Builder

ENV ...

# OS dependencies
USER root
RUN apk add --update --no-cache \
    ...

WORKDIR $APP_HOME

# Only copy the Gemfiles without any other source code so that we can install the gems
COPY Gemfile $APP_HOME/Gemfile
COPY Gemfile.lock $APP_HOME/Gemfile.lock

# Bundle install
RUN gem install bundler:2.4.10 && \
    ...

###############################
# Stage Final
###############################

FROM gcr.io/staging-artifacts-... as Production

ENV ...

USER root

RUN ...

# copy app with gems from former build stage
COPY --from=Builder /usr/local/bundle/ /usr/local/bundle/

WORKDIR $APP_HOME

# copy the app's source code
COPY --chown=my_project . $APP_HOME
# copy the gems from the builder image
COPY --from=Builder --chown=my_project $APP_HOME/vendor/bundle/ $APP_HOME/vendor/bundle/

USER my_project