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Rails 6 with Docker

All about running Rails 6 with Docker


Running Rails with Docker is a great solution for both development (eliminates any reliance on local system dependencies) and for deployment (no ruby or rails specific dependencies for operations to deal with).

These notes cover a few examples of running Rails 6 application in Docker, with solutions for some common Rails-specific refinements.

Running docker-compose-rails-6 example

The repo is a simple pre-fab example. Runs just as advertized…

git clone
cd docker-compose-rails-6
docker-compose run --rm web bundle install
docker-compose run --rm web yarn install
docker-compose up
docker-compose exec web ./bin/rails db:create

And it is running OK on http://localhost:3000 ..


Cleaning up..

$ docker-compose down
Removing docker-compose-rails-6_web_1 ... done
Removing docker-compose-rails-6_db_1  ... done
Removing network docker-compose-rails-6_default

A couple of things to note about this configuration:

  • the app folder is mapped to local file system
  • bundle and yarn installs assets in vender/bundle and node_modules i.e. these are not shipped in the image. This is not really ideal for deployment, as runtime dependencies could change from test and build-time.

Building a New Image From a Local Installation

Let’s start from scratch, with the basic aim of having a rails application that can:

  • run in the local OS as normal for development and testing
  • run in docker for development and testing

At the time of writing, I’m using Ruby 2.7.1 and Rails

Step 1: Build a Basic App (Local)

Checking Pre-requisites and Installation

$ node -v
$ npm -v
$ ruby -v
ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-darwin17]
$ gem install rails -v
$ rails --version
$ docker --version
Docker version 19.03.12, build 48a66213fe

Out-of-the-box application generation, built locally and using a local PostgreSQL database:

rails new minime --database=postgresql
cd minime

The rails new takes care of dependency installation. To recreate or install elsewhere, install dependencies:

bundle install
yarn install

Create and migrate the database, then fire up the app…

$ rails db:create
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
Created database 'minime_development'
Created database 'minime_test'

NB: get rid of the tzinfo-data platform warning with bundle config --local disable_platform_warnings true

$ rails db:migrate
$ rails server
=> Booting Puma
=> Rails application starting in development
=> Run `rails server --help` for more startup options
Puma starting in single mode...
* Version 4.3.6 (ruby 2.7.1-p83), codename: Mysterious Traveller
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://
* Listening on tcp://[::1]:3000
* Listening on tcp://
* Listening on tcp://
Use Ctrl-C to stop

And it is running OK on http://localhost:3000 ..


Adding a few more features (including tests)

rails generate controller Articles
rails generate model Article title:string text:text
rails db:migrate

And some tests:

$ rails test
Running via Spring preloader in process 57084
Run options: --seed 26227

# Running:


Finished in 0.439060s, 2.2776 runs/s, 2.2776 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Step 2: Adding Docker Support



docker-compose build
docker-compose run --rm web bundle exec rails db:create
docker-compose run --rm web bundle exec rails db:migrate
docker-compose up

Or to daemonize:

docker-compose up

And it is running OK on ..


Run a shell:

docker-compose run --rm web bash

Run tests in docker:

$ docker-compose run --rm web bundle exec rails test
Starting minime_db_1 ... done
Running via Spring preloader in process 15
Run options: --seed 19759

# Running:


Finished in 9.117707s, 0.1097 runs/s, 0.1097 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Rails-specific Refinements

Sharing the File System

For development, it can be useful to share the file system, so that editing locally can be immediately picked up by the container.

However, care is required with folders that may contain platform-specific versions. This is specifically an issue with node_modules.

The following volume mapping in the docker-compose.yml shares the application files but excludes node_modules.

    - .:/app
    - /app/node_modules/

If file sharing is not required, this volumes section can be removed (speeds things up).

Testing the shared file system by running localhost:3000/articles from docker and modifying the view file on the local file system:


Environment-driven Database Configuration

Updating the config/database.yml to ensure that the database configuration can be completely driven from the environment (and the docker-compose.yml file).

default: &default
  adapter: postgresql
  encoding: unicode
  host: <%= ENV['RAILS_DB_HOST'] %>
  username: <%= ENV['RAILS_DB_USERNAME'] %>
  password: <%= ENV['RAILS_DB_PASSWORD'] %>
  # For details on connection pooling, see Rails configuration guide
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  database: <%= ENV['RAILS_DB_NAME'] || "minime_#{ENV['RAILS_ENV'] || 'development'}" %>

NB: alternatively, use the standard DATABASE_URL environment settings, as described in the Rails guides.

Credits and References

About LCK#143 dockerrubyrails
Project Source on GitHub Return to the Project Catalog

This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

LittleCodingKata is my collection of programming exercises, research and code toys broadly spanning things that relate to programming and software development (languages, frameworks and tools).

These range from the trivial to the complex and serious. Many are inspired by existing work and I'll note credits and references where applicable. The focus is quite scattered, as I variously work on things new and important in the moment, or go back to revisit things from the past.

This is primarily a personal collection for my own edification and learning, but anyone who stumbles by is welcome to borrow, steal or reference the work here. And if you spot errors or issues I'd really appreciate some feedback - create an issue, send me an email or even send a pull-request.