rails app multi stage docker builds

Dockerfile

# Start from a small, trusted base image with the version pinned down
FROM ruby:2.5-slim-stretch AS base

# Install system dependencies required both at runtime and build time
# The image uses Postgres but you can swap it with mariadb-dev (for MySQL) or sqlite-dev
RUN apt-get update && \
    apt-get install --no-install-recommends -y \
      build-essential \
      # git is required for installing gems from git repos
      git \
      libpq-dev \
      tmpreaper \
      # libjemalloc1 (v3) provides big memory savings vs jemalloc v5+ (default on debian buster)
      libjemalloc1 \
      tmpreaper \
      && \
    apt-get clean && rm -rf /var/lib/apt/lists/*

ENV LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.1

FROM base AS builder

WORKDIR /rails_app

ADD ./Gemfile /rails_app/
ADD ./Gemfile.lock /rails_app/

RUN bundle config --global jobs `cat /proc/cpuinfo | grep processor | wc -l | xargs -I % expr % - 1`
RUN bundle install --without development test

ARG REVISION=''
ENV REVISION=$REVISION

RUN (cd /rails_app && echo $REVISION > commit_id.txt)
RUN (cd /rails_app && mkdir -p tmp/pids && rm -f tmp/pids/*.pid)

# debian stretch has libjemalloc1 https://packages.debian.org/stretch/libjemalloc1
FROM base
WORKDIR /rails_app
# We copy over the entire gems directory for our builder image, containing the already built artifact
COPY --from=builder /usr/local/bundle/ /usr/local/bundle/
COPY --from=builder /rails_app/ /rails_app/

# set a default RAILS_ENV for the build scripts
# this is required for the `rake assets:precompile` script
# to write assets to target dir set in `config.assets.prefix`
ARG RAILS_ENV=production
ENV RAILS_ENV=$RAILS_ENV

ADD ./ /rails_app

RUN (cd /rails_app && SECRET_KEY_BASE=1a bundle exec rake assets:precompile)

EXPOSE 81

CMD ["/rails_app/scripts/docker/start.sh"]