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"]

I see you have a bunch of environment variables with dummy values in the assets precompilation line. Are you using ENV.fetch in production.rb? If so, you can create custom environment, name it like building.rb, copy production.rb in it with no lines using ENV.fetch.
Then use it like RUN RAILS_ENV=building bundle exec rails assets:precompile

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