Sample of multistage Dockerfile for Rails app in production

Dockerfile

FROM madnight/docker-alpine-wkhtmltopdf as wkhtmltopdf_savior

# STAGE for bundle & yarn install
FROM ruby:2.4.3-alpine3.7 as builder

ENV CA_CERTS_PATH /etc/ssl/certs/
ENV RAILS_ENV production
ENV RAILS_LOG_TO_STDOUT true
ENV RAILS_SERVE_STATIC_FILES true

RUN apk add --no-cache \
  build-base \
  busybox \
  ca-certificates \
  curl \
  git \
  gnupg1 \
  graphicsmagick \
  libffi-dev \
  libsodium-dev \
  nodejs=8.9.3-r0 \
  openssh-client \
  postgresql-dev \
  rsync \
  yarn

RUN mkdir -p /app
WORKDIR /app

COPY Gemfile Gemfile.lock /app/
ARG SSH_CHECKOUT_KEY
RUN mkdir /root/.ssh/ \
  && echo "${SSH_CHECKOUT_KEY}" > /root/.ssh/id_rsa \
  && chmod 400 /root/.ssh/id_rsa \
  && touch /root/.ssh/known_hosts \
  && echo  $'\
github.com,192.30.255.112 ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ== \n\
bitbucket.org,104.192.143.1 ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAubiN81eDcafrgMeLzaFPsw2kNvEcqTKl/VqLat/MaB33pZy0y3rJZtnqwR2qOOvbwKZYKiEO1O6VqNEBxKvJJelCq0dTXWT5pbO2gDXC6h6QDXCaHo6pOHGPUy+YBaGQRGuSusMEASYiWunYN0vCAI8QaXnWMXNMdFP3jHAJH0eDsoiGnLPBlBp4TNm6rYI74nMzgz3B9IikW4WVK+dc8KZJZWYjAuORU3jc1c/NPskD2ASinf8v3xnfXeukU0sJ5N6m5E8VLjObPEO+mN2t/FZTMZLiFqPWc/ALSqnMnnhwrNi2rbfg/rd/IpL8Le3pSBne8+seeFVBoGqzHM9yXw== \n\
  ' >> /root/.ssh/known_hosts \
  && bundle config --global frozen 1 \
  && bundle install --without development test -j4 --retry 3 \
  && rm -rf /root/.ssh/id_rsa \
    /usr/local/bundle/bundler/gems/*/.git \
    /usr/local/bundle/cache/

COPY package.json yarn.lock /app/
RUN yarn install

COPY . /app/
RUN bundle exec rails \
  AWS_ACCESS_KEY_ID=dontneed \
  AWS_SECRET_ACCESS_KEY=dontneed \
  DATABASE_URL=postgresql:does_not_exist \
  SECRET_KEY_BASE=nein \
  assets:precompile

# Packaging final app w/o node_modules & the development tools
FROM ruby:2.4.3-alpine3.7

ENV CA_CERTS_PATH /etc/ssl/certs/
ENV RAILS_ENV production
ENV RAILS_SERVE_STATIC_FILES true
ENV RAILS_LOG_TO_STDOUT true

RUN apk add --no-cache \
  busybox \
  ca-certificates \
  curl \
  gnupg1 \
  graphicsmagick \
  libsodium-dev \
  nodejs=8.9.3-r0 \
  postgresql-dev \
  rsync 

# Patch for wkhtmltopdf
RUN apk add --update --no-cache \
    libgcc libstdc++ libx11 glib libxrender libxext libintl \
    libcrypto1.0 libssl1.0 \
    ttf-dejavu ttf-droid ttf-freefont ttf-liberation ttf-ubuntu-font-family

COPY --from=wkhtmltopdf_savior /bin/wkhtmltopdf /bin/

RUN mkdir -p /app
WORKDIR /app

COPY --from=builder /usr/local/bundle/ /usr/local/bundle/
COPY --from=builder /app/ /app/
COPY --from=builder /app/config/gpg/ /root/.gnupg/

EXPOSE 3000
CMD ["bundle", "exec", "rails", "server", "-b", "0.0.0.0"]

Thanks for this. I would add rm -rf node_modules tmp/* log/* just after assets precompile - that saves some space and copy time.