This is a living code example (step-by-step) inspired by https://docs.docker.com/compose/rails

This will repo will provide commit points so that it will be easy to checkout each step to follow along in case mistakes are made. This example was done on a MacBook with Docker version 18.01. Your example may differ based on platform.

docker build -t ruby-on-rails .

docker run -it -v "${PWD}:/app" ruby-on-rails /bin/bash

While running interactively in the Docker container, we're going to install the rails gem.

gem install rails -v 5.0.0.1

rails new app --force --database=postgresql

You should now have rails installed and a new rails application. You may exit the container by typing exit.

Now we can finish our Dockerfile and build our image to run our application.

docker build -t ruby-on-rails .

docker run --name web -p 3000:3000 ruby-on-rails bundle exec rails s -p 3000 -b '0.0.0.0'

This results in an error though because we did not configure the PostgreSQL database connection properly. We need to add the following to our config/database.yml:

default: &default
  adapter: postgresql
  encoding: unicode
  host: db
  username: postgres
  password:
  pool: 5

Also, instead of forcing ourselves to rebuild our container each time, we will mount our code directly into the container.

docker run -v "${PWD}:/myapp" --name web -p 3000:3000 ruby-on-rails bundle exec rails s -p 3000 -b '0.0.0.0'

We encounter another error though because we are not running our PostgreSQL database container.

docker run --name db postgres

docker run -v "${PWD}:/myapp" --name web --link db -p 3000:3000 ruby-on-rails bundle exec rails s -p 3000 -b '0.0.0.0'

Now we get a database does not exist error. We can fix that by running the rake db:create in our web container.

docker exec web rake db:create

Finally, we have a fully functioning Ruby on Rails application.