Disclaimer: this post is based on Quickstart: Compose and Rails — Docker the main difference is that I will run everything with my own user and cache the gems installations.

First, create the directory for your project:

% mkdir ~/my-blog
% cd ~/my-blog

Now create the Dockerfile:

from ruby

env DEBIAN_FRONTEND=noninteractive \
  NODE_VERSION=6.9.1

run sed -i '/deb-src/d' /etc/apt/sources.list && \
  apt-get update && \
  apt-get install -y build-essential libsqlite3-dev sqlite3

run curl -sSL "https://nodejs.org/dist/v$NODE_VERSION/node-v$NODE_VERSION-linux-x64.tar.xz" | tar xfJ - -C /usr/local --strip-components=1 && \
  npm install npm -g

run useradd -m -s /bin/bash -u 1000 railsuser
user railsuser

workdir /app

And the docker-compose.yml:

version: "2"

services:
  web:
    build: .
    volumes:
      - .:/app
      - bundle_path:/bundle
    environment:
      - BUNDLE_PATH=/bundle/vendor
      - BUNDLE_APP_CONFIG=/app/.bundle
    user: railsuser
    command: bundle exec rails server
    ports:
      - "3000:3000"

volumes:
  bundle_path:

Build it:

% docker-compose build

Create the Gemfile:

source "https://rubygems.org"

gem "rails"

Install your gems:

% docker-compose run --rm -u root web bash -c "mkdir -p /bundle/vendor && chown -R railsuser /bundle/vendor"
% docker-compose run --rm web bundle install --path /bundle/vendor

Build the project:

% docker-compose run web bundle exec rails new . --force --skip-bundle

Run bundle install again:

% docker-compose run --rm web bundle install

Bonus: Change the Project to use PostgreSQL

Edit Gemfile removing the gem sqlite3 and adding pg.

Run bundle install to install the gem pg:

docker-compose run --rm web bundle install

Change the Dockerfile to use PostgreSQL:

from ruby

env DEBIAN_FRONTEND=noninteractive \
  NODE_VERSION=6.9.1

run sed -i '/deb-src/d' /etc/apt/sources.list && \
  apt-get update && \
  apt-get install -y build-essential libpq-dev postgresql-client

run curl -sSL "https://nodejs.org/dist/v$NODE_VERSION/node-v$NODE_VERSION-linux-x64.tar.xz" | tar xfJ - -C /usr/local --strip-components=1 && \
  npm install npm -g

run useradd -m -s /bin/bash -u 1000 railsuser
user railsuser

workdir /app

Build it again:

% docker-compose build

Change docker-compose.yml to also run PostgreSQL:

version: "2"

services:
  web:
    build: .
    volumes:
      - .:/app
      - bundle_path:/bundle
    environment:
      - BUNDLE_PATH=/bundle/vendor
      - BUNDLE_APP_CONFIG=/app/.bundle
    user: railsuser
    command: bundle exec rails server -b 0.0.0.0
    ports:
      - "3000:3000"
    links:
      - db

  db:
    image: postgres
    volumes:
      - db:/var/lib/postgresql/data
    expose:
      - '5432'

volumes:
  bundle_path:
  db:

Update the configuration for the database editing the file config/database.yml like this:

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

test:
  <<: *default
  database: myblog_test

production:
  <<: *default
  database: myblog_production

Create your database, migrate and/or seed it:

docker-compose run --rm web bundle exec rake db:create
docker-compose run --rm web bundle exec rake db:migrate
docker-compose run --rm web bundle exec rake db:seed

#protip: you can execute rails db:setup to run altogether.

Now your project is ready to run your app with PostgreSQL.

At last but not least important: Document your up and running process

Remember: the process is now partially automated due to the bundler cache, so everyone will need to run these commands to setup the project:

% docker-compose run --rm -u root web bash -c "mkdir -p /bundle/vendor && chown railsuser /bundle/vendor"
% docker-compose run --rm web bundle install

And, of course:

% docker-compose run --rm web bundle exec rake db:setup

And to run:

% docker-compose up web

You can create a README like this:

# My Blog

## Running

Setup:

    % docker-compose run --rm -u root web bash -c "mkdir -p /bundle/vendor && chown railsuser /bundle/vendor"
    % docker-compose run --rm web bundle install
    % docker-compose run --rm web bundle exec rake db:setup

Run:

    % docker-compose up web

And go to [localhost:3000](http://localhost:3000).

Or even a bin/setup:

#!/bin/bash

set -xe

# Uncomment this line and ensure your config/database.yml.sample is ready to run with docker.
# cp config/database.yml.sample config/database.yml

docker-compose run --rm -u root web bash -c "mkdir -p /bundle/vendor && chown railsuser /bundle/vendor"
docker-compose run --rm web bundle install
docker-compose run --rm web bundle exec rake db:setup