Dockerfile with optimized Gem installs

Dockerfile

# The prepare stage extracts the versions of some heavy gems into
# individual files enabling us to preinstall these gems in the build stage
# below. Thus we won't have to install the heavy gems everytime the
# Gemfile.lock changes, only when heavy gem versions are changed.
FROM ruby:2.6.3 AS prepare

RUN mkdir /gem_versions
COPY Gemfile.lock /gem_versions/Gemfile.lock
RUN grep "^    nokogiri " /gem_versions/Gemfile.lock | awk -F '[()]' '{print $2}' > /gem_versions/nokogiri.version
RUN grep "^    sassc " /gem_versions/Gemfile.lock | awk -F '[()]' '{print $2}' > /gem_versions/sassc.version

## Build stage
FROM ruby:2.6.3 AS build

WORKDIR /app

COPY --from=prepare /gem_versions/sassc.version /gem_versions/
RUN gem install sassc -v $(cat /gem_versions/sassc.version)

COPY --from=prepare /gem_versions/nokogiri.version /gem_versions/
RUN gem install nokogiri -v $(cat /gem_versions/nokogiri.version)

COPY Gemfile Gemfile.lock /app/
RUN bundle install --jobs=4 --clean