Deploy Rails app into Heroku with Docker using heroku.yml - ruby-on-rails

Question

After following the tutorial at: https://devcenter.heroku.com/articles/build-docker-images-heroku-yml it was not clear which contents we should put in the Dockerfile.
For a Rails app what do we need in our Dockerfile?

Answer 1

There is a Rails 5 example that you can base your solution: https://github.com/jahangiranwari/rails5-docker-heroku/
For a Rails 6 project I've used the following Dockerfile:
FROM ruby:2.6.6
# Install node & yarn
RUN curl -sL https://deb.nodesource.com/setup_12.x | bash -
RUN apt-get install -y nodejs
RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
RUN apt-get update && apt-get install -y yarn
# Install base deps or additional (e.g. tesseract)
ARG INSTALL_DEPENDENCIES
RUN apt-get update -qq \
&& apt-get install -y --no-install-recommends ${INSTALL_DEPENDENCIES} \
build-essential libpq-dev git \
&& apt-get clean autoclean \
&& apt-get autoremove -y \
&& rm -rf \
/var/lib/apt \
/var/lib/dpkg \
/var/lib/cache \
/var/lib/log
# Install deps with bundler
RUN mkdir /app
WORKDIR /app
COPY Gemfile* /app/
ARG BUNDLE_INSTALL_ARGS
RUN gem install bundler:2.1.4
RUN bundle config set without 'development test'
RUN bundle install ${BUNDLE_INSTALL_ARGS} \
&& rm -rf /usr/local/bundle/cache/* \
&& find /usr/local/bundle/gems/ -name "*.c" -delete \
&& find /usr/local/bundle/gems/ -name "*.o" -delete
COPY . /app/
# Compile assets
ARG RAILS_ENV=development
RUN if [ "$RAILS_ENV" = "production" ]; then SECRET_KEY_BASE=$(rake secret) bundle exec rake assets:precompile; fi
And this heroku.yml file:
build:
docker:
web: Dockerfile
config:
BUNDLE_INSTALL_ARGS: --jobs 10 --retry=3
RAILS_ENV: production
# Put extra deps here
INSTALL_DEPENDENCIES: curl openssh-server python
run:
web: bundle exec puma -C config/puma.rb
Update 20-1-22:
It works for Rails 7