A basic Dockerfile for a Rails app moving off Heroku, without addons

Dockerfile

# Our base image is Ruby 2.2; it will run on Amazon Linux.
FROM ruby:2.2

# Install packages
RUN apt-get update && apt-get install -y \
    git \
    nodejs \
    tzdata

# Copy your application into the container.
COPY . .

# Build your application.
RUN \
    # Install application gems.
    bundle install --jobs 4 --without development test --with production && \
    # Precompile Rails assets.
    RAILS_ENV=production bundle exec rake assets:precompile && \
    # Clean up build package
    rm -rf /usr/local/lib/ruby/gems/*/cache/* && \
    rm -rf ~/.gem

# Run your application.
CMD bin/rails server --port 3000

To move off Heroku:

• Add the Dockerfile to the root of the application; for a plain Rails app, this will work, adjusting this for Ruby versions
• Add a new environment to database.yml; it'll be something like this:

production:
  adapter: mysql2
  encoding: utf8
  database: <app_name>_prod
  username: root

• Spin up new servers, on your own, or using a service; I'm partial to Opsolutely
• Snapshot the database in Heroku, using Heroku's backup commands
• Initialize new database instances; like Amazon's RDS or Amazon's Aurora (which has improved performance and replication)
• When the new server and database are setup, update DNS to point traffic to the new servers
• Decommission the old servers