Why Alpine Linux?

When you Dockerize a Rails application, you have to choose which distribution of Linux you want your container to use.

When I first started Dockerizing my applications I used Ubuntu for my containers because that was the distro I was familiar with. Unfortunately I discovered that using Ubuntu results in slow builds, slow running of commands, and large image sizes.

I discovered that Alpine Linux is popular for Docker containers because it affords better performance and smaller images.

Alpine + Capybara problems

Alpine had its own drawback though: I couldn’t run my tests because it wasn’t as straightforward in Alpine to get a Capybara + ChromeDriver configuration working on Alpine.

The evident reason for this is that Alpine can’t run a normal Chrome package the way Ubuntu can. Instead, it’s typical on Alpine to use Chromium, which doesn’t quite play nice with Capybara the way Chrome does.

How to get Alpine + Capyabara working

There are three steps to getting Capybara working on Alpine.

1. Use selenium-webdriver instead of webdrivers
2. Install chromium, chromium-chromedriver and selenium on your Docker image
3. Configure a Capybara driver

Use selenium-webdriver instead of webdrivers

The first step is very simple: if you happen to be using the webdrivers gem in your Gemfile, replace it with selenium-webdriver.

Install chromium, chromium-chromedriver and selenium on your Docker image

The next step is to alter your Dockerfile so that chromium, chromium-chromedriver and selenium are installed.

Below is a Dockerfile from one of my projects (which is based on Mike Rogers’ fantastic Docker-Rails template).

I’ll call out the relevant bits of the file.

chromium chromium-chromedriver python3 python3-dev py3-pip

This line, as you can see, installs chromium and chromium-chromedriver. It also installs pip3 and its dependencies because we need pip3 in order to install Selenium. (If you don’t know, pip3 is a Python package manager.)

Here’s the line that installs Selenium:

RUN pip3 install -U selenium

And here’s the full Dockerfile.

FROM ruby:2.7.2-alpine AS builder

RUN apk add --no-cache \
    build-base libffi-dev \
    nodejs yarn tzdata \
    postgresql-dev postgresql-client zlib-dev libxml2-dev libxslt-dev readline-dev bash \
    #
    # For testing
    chromium chromium-chromedriver python3 python3-dev py3-pip \
    #
    # Nice-to-haves
    git vim \
    #
    # Fixes watch file issues with things like HMR
    libnotify-dev

RUN pip3 install -U selenium

FROM builder AS development

# Add the current apps files into docker image
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

# Install any extra dependencies via Aptfile - These are installed on Heroku
# COPY Aptfile /usr/src/app/Aptfile
# RUN apk add --update $(cat /usr/src/app/Aptfile | xargs)

ENV PATH /usr/src/app/bin:$PATH

# Install latest bundler
RUN bundle config --global silence_root_warning 1

EXPOSE 3000
CMD ["rails", "server", "-b", "0.0.0.0", "-p", "3000"]

FROM development AS production

COPY Gemfile /usr/src/app
COPY .ruby-version /usr/src/app
COPY Gemfile.lock /usr/src/app

COPY package.json /usr/src/app
COPY yarn.lock /usr/src/app

# Install Ruby Gems
RUN bundle config set deployment 'true'
RUN bundle config set without 'development:test'
RUN bundle check || bundle install --jobs=$(nproc)

# Install Yarn Libraries
RUN yarn install --check-files

# Copy the rest of the app
COPY . /usr/src/app

# Precompile the assets
RUN RAILS_SERVE_STATIC_FILES=enabled SECRET_KEY_BASE=secret-key-base RAILS_ENV=production RACK_ENV=production NODE_ENV=production bundle exec rake assets:precompile

# Precompile Bootsnap
run RAILS_SERVE_STATIC_FILES=enabled SECRET_KEY_BASE=secret-key-base RAILS_ENV=production RACK_ENV=production NODE_ENV=production bundle exec bootsnap precompile --gemfile app/ lib/

The next step is to configure a Capybara driver.

Configure a Capybara driver

Below is the Capybara configuration that I use for one of my projects. This configuration is actually identical to the config I used before I started using Docker, so there’s nothing special here related to Alpine Linux, but the Alpine Linux configuration described in this post won’t work without something like this.

# spec/support/chrome.rb

driver = :selenium_chrome_headless

Capybara.server = :puma, {Silent: true}

Capybara.register_driver driver do |app|
  options = ::Selenium::WebDriver::Chrome::Options.new

  options.add_argument("--headless")
  options.add_argument("--no-sandbox")
  options.add_argument("--disable-dev-shm-usage")
  options.add_argument("--window-size=1400,1400")

  Capybara::Selenium::Driver.new(app, browser: :chrome, options: options)
end

Capybara.javascript_driver = driver

RSpec.configure do |config|
  config.before(:each, type: :system) do
    driven_by driver
  end
end

Remember to make sure that the line in spec/rails_helper that includes files from spec/support is uncommented so this file gets loaded.