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Getting started with Docker Compose and Rails
documentation, docs, docker, compose, orchestration, containers
Quickstart: Compose and Rails

This Quickstart guide will show you how to use Docker Compose to set up and run a Rails/PostgreSQL app. Before starting, you'll need to have Compose installed.

Define the project

Start by setting up the three files you'll need to build the app. First, since your app is going to run inside a Docker container containing all of its dependencies, you'll need to define exactly what needs to be included in the container. This is done using a file called Dockerfile. To begin with, the Dockerfile consists of:

FROM ruby:2.3.3
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
RUN mkdir /myapp
WORKDIR /myapp
ADD Gemfile /myapp/Gemfile
ADD Gemfile.lock /myapp/Gemfile.lock
RUN bundle install
ADD . /myapp

That'll put your application code inside an image that will build a container with Ruby, Bundler and all your dependencies inside it. For more information on how to write Dockerfiles, see the Docker user guide and the Dockerfile reference.

Next, create a bootstrap Gemfile which just loads Rails. It'll be overwritten in a moment by rails new.

source 'https://rubygems.org'
gem 'rails', '5.0.0.1'

You'll need an empty Gemfile.lock in order to build our Dockerfile.

touch Gemfile.lock

Finally, docker-compose.yml is where the magic happens. This file describes the services that comprise your app (a database and a web app), how to get each one's Docker image (the database just runs on a pre-made PostgreSQL image, and the web app is built from the current directory), and the configuration needed to link them together and expose the web app's port.

version: '2'
services:
  db:
    image: postgres
  web:
    build: .
    command: bundle exec rails s -p 3000 -b '0.0.0.0'
    volumes:
      - .:/myapp
    ports:
      - "3000:3000"
    depends_on:
      - db

Tip: You can use either a .yml or .yaml extension for this file.

Build the project

With those three files in place, you can now generate the Rails skeleton app using docker-compose run:

docker-compose run web rails new . --force --database=postgresql --skip-bundle

First, Compose will build the image for the web service using the Dockerfile. Then it'll run rails new inside a new container, using that image. Once it's done, you should have generated a fresh app:

  $ ls -l
  total 56
  -rw-r--r--   1 user  staff   215 Feb 13 23:33 Dockerfile
  -rw-r--r--   1 user  staff  1480 Feb 13 23:43 Gemfile
  -rw-r--r--   1 user  staff  2535 Feb 13 23:43 Gemfile.lock
  -rw-r--r--   1 root  root   478 Feb 13 23:43 README.rdoc
  -rw-r--r--   1 root  root   249 Feb 13 23:43 Rakefile
  drwxr-xr-x   8 root  root   272 Feb 13 23:43 app
  drwxr-xr-x   6 root  root   204 Feb 13 23:43 bin
  drwxr-xr-x  11 root  root   374 Feb 13 23:43 config
  -rw-r--r--   1 root  root   153 Feb 13 23:43 config.ru
  drwxr-xr-x   3 root  root   102 Feb 13 23:43 db
  -rw-r--r--   1 user  staff   161 Feb 13 23:35 docker-compose.yml
  drwxr-xr-x   4 root  root   136 Feb 13 23:43 lib
  drwxr-xr-x   3 root  root   102 Feb 13 23:43 log
  drwxr-xr-x   7 root  root   238 Feb 13 23:43 public
  drwxr-xr-x   9 root  root   306 Feb 13 23:43 test
  drwxr-xr-x   3 root  root   102 Feb 13 23:43 tmp
  drwxr-xr-x   3 root  root   102 Feb 13 23:43 vendor

If you are running Docker on Linux, the files rails new created are owned by root. This happens because the container runs as the root user. Change the ownership of the new files.

  sudo chown -R $USER:$USER .

If you are running Docker on Mac or Windows, you should already have ownership of all files, including those generated by rails new. List the files just to verify this.

If you edit Gemfile at this point or later, you will need to build the image again. (This, and changes to the Dockerfile itself, should be the only times you'll need to rebuild.)

docker-compose build

Connect the database

The app is now bootable, but you're not quite there yet. By default, Rails expects a database to be running on localhost - so you need to point it at the db container instead. You also need to change the database and username to align with the defaults set by the postgres image.

Replace the contents of config/database.yml with the following:

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

test:
  <<: *default
  database: myapp_test

You can now boot the app with:

docker-compose up

If all's well, you should see some PostgreSQL output, and then—after a few seconds—the familiar refrain:

myapp_web_1 | [2014-01-17 17:16:29] INFO  WEBrick 1.3.1
myapp_web_1 | [2014-01-17 17:16:29] INFO  ruby 2.2.0 (2014-12-25) [x86_64-linux-gnu]
myapp_web_1 | [2014-01-17 17:16:29] INFO  WEBrick::HTTPServer#start: pid=1 port=3000

Finally, you need to create the database. In another terminal, run:

docker-compose run web rails db:create

That's it. Your app should now be running on port 3000 on your Docker daemon. If you're using Docker Machine, then docker-machine ip MACHINE_VM returns the Docker host IP address.

Rails example

Note: If you stop the example application and attempt to restart it, you might get the following error: web_1 | A server is already running. Check /myapp/tmp/pids/server.pid. One way to resolve this is to delete the file tmp/pids/server.pid, and then re-start the application with docker-compose up.

More Compose documentation