Ruby on Rails Deployment Image: Builds a ready to be deployed image of a Ruby on Rails application.

Dockerfile

# Ruby on Rails Deployment Image
#
# Builds a ready to be deployed image of a Ruby on Rails application.
# https://ebababi.net/dockerfile-for-ruby-on-rails-deployments.html

# Copyright (C) 2019 Nikolaos Anastopoulos, Inc. All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions
# are met:
# 1. Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
# 2. Redistributions in binary form must reproduce the above copyright
# notice, this list of conditions and the following disclaimer in the
# documentation and/or other materials provided with the distribution.
#
# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
# ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
# SUCH DAMAGE.


FROM ruby:2.4.5

# Install Ruby on Rails dependencies.
# https://guides.rubyonrails.org/development_dependencies_install.html#ubuntu
RUN apt-get update \
    && apt-get install -y --no-install-recommends \
        apt-transport-https \
        software-properties-common \
    && apt-key adv --keyserver "hkp://ipv4.pool.sks-keyservers.net" \
        --recv-key "9FD3B784BC1C6FC31A8A0A1C1655A0AB68576280" \
    && add-apt-repository "deb https://deb.nodesource.com/node_10.x stretch main" \
    && apt-key adv --keyserver "hkp://ipv4.pool.sks-keyservers.net" \
        --recv-key "72ECF46A56B4AD39C907BBB71646B01B86E50310" \
    && add-apt-repository "deb https://dl.yarnpkg.com/debian/ stable main" \
    && apt-get update \
    && apt-get install -y --no-install-recommends \
        nodejs \
        yarn \
        sqlite3 \
        libsqlite3-dev \
        mysql-client \
        default-libmysqlclient-dev \
        postgresql-client \
        libpq-dev \
        imagemagick \
        ffmpeg \
        poppler-utils \
    && rm -rf /var/lib/apt/lists/*

# Use the recommended working path.
WORKDIR /usr/src/app

# Set environment variables indicating a production build.
ENV RACK_ENV production
ENV RAILS_ENV production
ENV NODE_ENV production

# Install Ruby dependencies.
COPY Gemfile* ./
RUN bundle install --without development:test --frozen --no-cache

# Install JavaScript dependencies.
COPY package.json yarn.lock ./
RUN yarn install --frozen-lockfile --no-cache --production \
    && yarn cache clean

# Copy application code to the working path.
COPY . ./

# Set Rails environment variables appropriate to a Docker image.
ENV RAILS_LOG_TO_STDOUT enabled
ENV RAILS_SERVE_STATIC_FILES enabled
ENV REDIS_PROVIDER REDIS_URL

# Set default values to required Rails environment variables.
ARG SECRET_KEY_BASE=deca1fc2f5da822a699fffb01fffbf97bb736e9ec3720637
ARG DATABASE_URL=sqlite3::memory:

# Prepare for Rails asset pipeline.
RUN bin/rake assets:precompile \
    && bin/rake assets:clean

ENV PORT 3000
EXPOSE $PORT

CMD ["bin/rails", "server", "--binding=0.0.0.0"]

I usually keep the database as a separate docker, since we use Postgres and not SQLite.

You're right, @kp666, that's a good practice. The proposed Dockerfile does not imply a way to setup database connections. Either config/database.yml, or DATABASE_URL, or any other way can be used to do so. As a note though, someone might be confused with the DATABASE_URL set to sqlite3::memory: during build time. This is done because bin/rake assets:precompile requires a database connection present, although it is not using it and that "hack" allows for the image to build on systems without access to the database.

Also, I mount the master.key from a volume.

Yeap, that's important. In addition, keys files must be included in .dockerignore...

As a note though, someone might be confused with the DATABASE_URL set to sqlite3::memory: during build time. This is done because bin/rake assets:precompile requires a database connection present, although it is not using it and that "hack" allows for the image to build on systems without access to the database.

This is something I didn't know, that's a neat hack :)
What I was mentioning about the database been in a separate docker is that then I wouldn't have to install any of the databases on this docker image. But then my deployment is also different from your's as assets are precompiled at the CI for me.