Add docker

Dockerfile

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+FROM ruby:2.6
+
+RUN apt-get update
+RUN apt-get install software-properties-common -y
+
+# Setup additional repositories
+RUN apt-key adv --recv-keys --keyserver hkp://keyserver.ubuntu.com:80 0xF1656F24C74CD1D8
+RUN add-apt-repository 'deb [arch=amd64,i386,ppc64el] http://mirrors.coreix.net/mariadb/repo/10.1/ubuntu xenial main'
+RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
+RUN apt-get update
+
+# Install main dependencies
+RUN apt-get install -y \
+  build-essential  \
+  curl \
+  libmariadbclient-dev \
+  nano \
+  nodejs
+
+# Setup an application
+RUN useradd -r -d /opt/postal -m -s /bin/bash -u 999 postal
+USER postal
+RUN mkdir -p /opt/postal/app /opt/postal/config
+WORKDIR /opt/postal/app
+
+# Install bundler
+RUN gem install bundler --no-doc
+RUN bundle config frozen 1
+RUN bundle config build.sassc --disable-march-tune-native
+
+# Install the latest and active gem dependencies and re-run
+# the appropriate commands to handle installs.
+COPY Gemfile Gemfile.lock ./
+RUN bundle install -j 4
+
+# Copy the application (and set permissions)
+COPY --chown=postal . .
+
+# Copy temporary configuration file which can be used for
+# running the asset precompilation.
+COPY --chown=postal config/postal.defaults.yml /opt/postal/config/postal.yml
+
+# Precompile assets
+RUN POSTAL_SKIP_CONFIG_CHECK=1 RAILS_GROUPS=assets bundle exec rake assets:precompile
+RUN touch /opt/postal/app/public/assets/.prebuilt
+
+# Set the CMD
+CMD ["bundle", "exec"]