app.Dockerfile

######################
# Stage: Builder
FROM ruby:2.5-slim as Builder

# Alpine syntax:
#RUN apk add --update --no-cache \
#    build-base \
#    git \
#    nodejs-current \
#    postgresql-dev \
#    tzdata

RUN echo "deb http://ftp.sg.debian.org/debian/ stable main contrib non-free" > /etc/apt/sources.list \
    && echo "deb http://ftp.sg.debian.org/debian/ stable-updates main contrib non-free" >> /etc/apt/sources.list \
    && echo "deb http://security.debian.org/ stable/updates main" >> /etc/apt/sources.list \
    && echo "deb http://ftp.debian.org/debian stretch-backports main" >> /etc/apt/sources.list

# Debian syntax:
RUN apt-get update -qq \
    && apt-get install --no-install-recommends --fix-missing -y \
           build-essential \
           curl \
           git \
           gnupg \
           libpq-dev \
    && curl -sL https://deb.nodesource.com/setup_8.x | bash - \
    && apt-get install --no-install-recommends --fix-missing -y \
           nodejs \
    && rm -rf /var/lib/apt/lists/*

ARG arg_rails_root=/myapp
ARG arg_rails_env=production
ENV RAILS_ENV=$arg_rails_env

RUN mkdir -p $arg_rails_root
WORKDIR $arg_rails_root

# bundle gem dependencies
# next two steps will be cached unless Gemfile or Gemfile.lock changes.
# --jobs $(nproc) runs bundler in parallel with the amount of CPUs processes.
COPY Gemfile $arg_rails_root/Gemfile
COPY Gemfile.lock $arg_rails_root/Gemfile.lock

RUN bundle config --global frozen 1 \
    && bundle install --jobs $(nproc) --retry 5 --without development test \
    # Remove unneeded files (cached *.gem, *.o, *.c)
    && rm -rf /usr/local/bundle/cache/*.gem \
    && find /usr/local/bundle/gems/ -name "*.c" -delete \
    && find /usr/local/bundle/gems/ -name "*.o" -delete \
    && find /usr/local/bundle/gems/ -name "*.a" -delete

COPY . $arg_rails_root
RUN mv config/database.example.yml config/database.yml \
    && mv config/app_config.example.yml config/app_config.yml \
    && mv .env.example .env

# We compile the assets. When running the rake task, DATABASE_URL is required and we pass a dummy value.
RUN RAILS_ENV=$arg_rails_env SECRET_KEY_BASE=$(bundle exec rake secret) bundle exec rake DATABASE_URL=postgresql:does_not_exist assets:precompile \
    && rm -rf $arg_rails_root/node_modules \
       $arg_rails_root/tmp/cache \
       $arg_rails_root/app/assets \
       $arg_rails_root/vendor/assets \
       $arg_rails_root/lib/assets \
       $arg_rails_root/spec

# The tmp folder gets created during asset pre-compilation. We will remove it from the image because it will be mounted
# as a tmpfs volume. If the folder is already there, the mount does not work properly.
RUN rm -rf $arg_rails_root/tmp config/database.yml config/app_config.yml .env

###############################
# Stage Final
FROM ruby:2.5-slim

# Alpine syntax:
#RUN apk add --update --no-cache \
#    gettext \
#    postgresql-client \
#    tzdata

RUN echo "deb http://ftp.sg.debian.org/debian/ stable main contrib non-free" > /etc/apt/sources.list \
    && echo "deb http://ftp.sg.debian.org/debian/ stable-updates main contrib non-free" >> /etc/apt/sources.list \
    && echo "deb http://security.debian.org/ stable/updates main" >> /etc/apt/sources.list \
    && echo "deb http://ftp.debian.org/debian stretch-backports main" >> /etc/apt/sources.list

# Debian synatx:
RUN apt-get update -qq \
    && apt-get install --no-install-recommends --fix-missing -y \
           libpq-dev \
    && rm -rf /var/lib/apt/lists/*

ARG deploy_user=deploy
ARG arg_rails_root=/myapp
ARG arg_rails_env=production
ENV RAILS_ENV=$arg_rails_env
ENV RAILS_SERVE_STATIC_FILES true
ENV RAILS_LOG_TO_STDOUT true
ENV EXECJS_RUNTIME Disabled

# Create non privileged user, set ownership and change user
# Alpine syntax:
#RUN addgroup -g 1000 -S $deploy_user \
#    && adduser -u 1000 -S $deploy_user -G $deploy_user
# Debian syntax.
RUN addgroup --gid 1000 $deploy_user \
    && adduser --uid 1000 --gid 1000 --disabled-password --gecos "" $deploy_user
USER $deploy_user

# Copy app with gems from former build stage
COPY --from=Builder /usr/local/bundle/ /usr/local/bundle/
COPY --from=Builder --chown=deploy:deploy $arg_rails_root $arg_rails_root

WORKDIR $arg_rails_root

EXPOSE 3000

# Save timestamp of image building
RUN date -u > BUILD_TIME