database.yml

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

development:
  <<: *default
  database: db_development

test:
  <<: *default
  database: db_test

production:
  <<: *default
  database: db_production

docker-compose.yml

version: '2'
services:
  web:
    build: .
    command: bundle exec rails s -p 3000 -b '0.0.0.0'
    tty: true
    stdin_open: true
    volumes:
      - .:/app
      - ~/.ssh:/root/.ssh
    volumes_from:
      - box
    ports:
      - "3000:3000"
    depends_on:
      - db
  db:
    image: postgres:9.5
  box:
    image: busybox
    volumes:
      - /bundle

Dockerfile

FROM ruby:2.2.3

RUN apt-get update -qq
RUN apt-get install -y -qq build-essential
RUN apt-get install -y -qq nodejs
RUN apt-get install -y -qq libpq-dev

ENV APP_HOME /app
ENV BUNDLE_PATH /bundle
ENV BUNDLE_JOBS 5

RUN mkdir -p $APP_HOME

WORKDIR $APP_HOME

RUN gem install bundler --no-ri --no-rdoc