refactor: Allow stage 1 steps to be reused

Dockerfile

 FROM ruby:2.7.5-slim-bullseye
 
-RUN apt-get update \
-    && DEBIAN_FRONTEND=noninteractive apt-get \
-        -o Dpkg::Options::="--force-confnew" --allow-remove-essential --allow-change-held-packages -fuy \
-        upgrade \
-    && apt-get install -y --no-install-recommends \
-        cmake \
-        pkg-config \
-        default-libmysqlclient-dev \
-        nodejs \
-        curl \
-        git \
-        ruby-dev \
-        telnet \
-        nano \
-        build-essential \
-        libpq-dev \
-        shared-mime-info \
-    && apt-get autoremove \
-    && apt-get autoclean \
-    && apt-get clean \
-    && rm -rf /var/lib/apt/lists/* \
-    && adduser nexus --system --group -u 1000 && mkdir -p /app && chown -R nexus:nexus /app
+# Bootstrap scripts will also be used by stage 2 (rails-runtime-base-image).
+ADD /app/bootstrap*sh /app/
+RUN chmod +x /app/bootstrap*sh
 
-USER nexus
+# Install OS packages required for the gem build environment.
+RUN /app/bootstrap-apt-upgrade.sh
+RUN apt-get install -y --no-install-recommends \
+      build-essential \
+      cmake \
+      curl \
+      default-libmysqlclient-dev \
+      git \
+      libpq-dev \
+      nano \
+      nodejs \
+      pkg-config \
+      ruby-dev \
+      shared-mime-info \
+      telnet
+RUN /app/bootstrap-apt-clean.sh
 
+# Prepare bundle for the gem build environment.
+RUN /app/bootstrap-app-dir.sh
+USER nexus
 WORKDIR /app
 COPY --chown=nexus:nexus Gemfile Gemfile.lock ./
-RUN bundle config --local build.sassc --disable-march-tune-native && gem install bundler -v 2.1 && bundle install --jobs 20 --retry 5
+RUN /app/bootstrap-app-bundle.sh

README.md

 # Rails Base Image Builder
 
-This repository is the base image for all base image that is used in the `builder` stage of all of our rails apps.
+This repository is the 'stage 1' base image used in the `builder` stage on all of our rails applications.
+
+* Stage 1 (rails-base-image-builder): Install packages to compile gems, and compile those gems.
+* Stage 2 (rails-runtime-base-image): Install packages and configure OS for runtime environment.
 
 ## Update gems
 
-To update the gems in this core image, great care must be taken. Once the gemfile has been update, the lock file can be updated with
-a simple helper script. There are many ways of achieving this, but the recommended way is:
+To update the gems in this core image, great care must be taken. Once the gemfile has been update, the lock file can be
+updated with a simple helper script. There are many ways of achieving this, but the recommended way is:
 
 ```
 docker run -it -v "$(pwd)":/app -v /usr/local/bundle:/usr/local/bundle ruby:2.7.5 sh -c "apt-get update; apt-get install -y cmake build-essential; cd /app; gem install bundler:2.1; bundle update; bundle install;"

app/bootstrap-app-bundle.sh

+#!/bin/sh
+set -ex
+bundle config --local build.sassc --disable-march-tune-native
+gem install bundler -v 2.1
+bundle install --jobs 20 --retry 5

app/bootstrap-app-dir.sh

+#!/bin/sh
+set -ex
+# Common bootstrap for a nexus application in a container.
+adduser nexus --system --group -u 1000
+chown -R nexus:nexus /app

app/bootstrap-apt-clean.sh

+#!/bin/sh
+set -ex
+apt-get autoremove
+apt-get autoclean
+apt-get clean
+rm -rf /var/lib/apt/lists/*

app/bootstrap-apt-upgrade.sh

+#!/bin/sh
+set -ex
+apt-get update
+DEBIAN_FRONTEND=noninteractive apt-get \
+  -o Dpkg::Options::="--force-confnew" --allow-remove-essential --allow-change-held-packages -fuy \
+  upgrade