Docker for Rails

How to dockerize your application

This tutorial is meant to help anyone who struggles to set up a Rails environment with Docker. Here I’ll cover how to use Docker and docker-compose, which work together in perfect harmony.

A quick overview of Docker

Docker is container software from Docker, Inc. that provides a layer of abstraction and automation for operating-system-level virtualization on Windows, macOS, and Linux.

Requirements:

Docker installed on your machine: Installing Docker docker-compose installed: Installing docker-compose

Defining the project

Start by setting up the files needed to build the application that will run, along with its dependencies, inside a Docker container.

The files are:

Dockerfile

docker-compose.yml

Gemfile

Gemfile.lock

The first step is to create the Dockerfile

FROM ruby:3.0.0
RUN apt-get update -qq && apt-get install -y nodejs postgresql-client
RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - && \
  echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list && \
  apt-get update -qq && apt-get install -y nodejs postgresql-client vim && \
  apt-get install -y yarn && \
  apt-get install -y imagemagick && \
  apt-get install -y libvips-tools && \
  apt-get install -y locales

# Install our dependencies
RUN apt-get update && apt-get install -qq -y --no-install-recommends \
nodejs yarn build-essential libpq-dev imagemagick git-all nano

WORKDIR /yourapp
COPY Gemfile /yourapp/Gemfile
COPY Gemfile.lock /yourapp/Gemfile.lock
RUN bundle update mimemagic
RUN bundle install
COPY . /yourapp

This puts your application code inside an image that builds a container with Ruby, Bundler, and all of your dependencies.

Now let’s create the Gemfile

source 'https://rubygems.org'
gem 'rails', '~>6'

Like every Rails project, we also need a Gemfile.lock

touch Gemfile.lock

Then the docker-compose file:

version: "3.9"
services:
  db:
    image: postgres
    volumes:
      - ./tmp/db:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: password
  web:
    build: .
    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
    volumes:
      - .:/yourapp
    ports:
      - "3000:3000"
    depends_on:
      - db

Now let’s generate the project

docker-compose run --no-deps web rails new . --force --database=postgresql

First, Compose builds the image for the web service using the Dockerfile. The --no-deps flag tells Compose not to start linked services. Then it runs rails new inside a new container using that image. Once that’s done, you should have a freshly generated application.

You’ll need permission to edit the generated files

sudo chown -R $USER:$USER .

You now have the full Rails file structure in place. Next, we need to rebuild the image so we can use it with Docker.

docker-compose build

Database configuration

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

development:
  <<: *default
  database: myapp_development


test:
  <<: *default
  database: myapp_test

Now you can start the application with

docker-compose up

If everything is fine, this is the message you’ll see in the terminal

rails_db_1 is up-to-date
Creating rails_web_1 ... done
Attaching to rails_db_1, rails_web_1
db_1   | PostgreSQL init process complete; ready for start up.
db_1   |
db_1   | 2018-03-21 20:18:37.437 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432
db_1   | 2018-03-21 20:18:37.437 UTC [1] LOG:  listening on IPv6 address "::", port 5432
db_1   | 2018-03-21 20:18:37.443 UTC [1] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
db_1   | 2018-03-21 20:18:37.726 UTC [55] LOG:  database system was shut down at 2018-03-21 20:18:37 UTC
db_1   | 2018-03-21 20:18:37.772 UTC [1] LOG:  database system is ready to accept connections

Now let’s create the database


docker-compose run web rake db:create

The output will look like this

Starting rails_db_1... done
Created database 'yourapp_development'
Created database 'yourapp_test'

Then open your browser and go to


localhost:3000

References