Sample of Dockerfile and docker-compose.yml

docker-compose.yml

version: "2"

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

  web:
    build: .
    volumes:
      - .:/app
    ports:
      - "3000:3000"
    links:
      - db

Dockerfile

from ruby:2.3.0

env DEBIAN_FRONTEND noninteractive

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

env PHANTOMJS_VERSION 1.9.8

run wget https://bitbucket.org/ariya/phantomjs/downloads/phantomjs-$PHANTOMJS_VERSION-linux-x86_64.tar.bz2 -O- | tar xf
run ln -s /usr/local/phantomjs-$PHANTOMJS_VERSION-linux-x86_64/bin/phantomjs /usr/local/bin/phantomjs

workdir /tmp
copy Gemfile Gemfile
copy Gemfile.lock Gemfile.lock

run bundle install

workdir /app

cmd ["rails", "server", "-b", "0.0.0.0"]