capybara.rb

require "selenium-webdriver"
require "capybara"

Capybara.register_driver :containerized_selenium do |app|
  Capybara::Selenium::Driver.new(
    app,
    browser: :remote,
    url: "http://selenium:4444/wd/hub",
    desired_capabilities: Selenium::WebDriver::Remote::Capabilities.firefox
  )
end

web = Capybara::Session.new(:containerized_selenium)
web.visit("http://example.com")
puts web.html

docker-compose.yml

version: '3.3'

services:
  selenium:
    image: selenium/standalone-firefox-debug
    volumes:
      # The images need access to the hosts' shared memory;
      # see https://github.com/SeleniumHQ/docker-selenium#running-the-images
      - /dev/shm:/dev/shm
    expose:
      - 4444
      - 9515

  spec:
    image: ruby
    links:
      - selenium

selenium_webdriver.rb

require "selenium-webdriver"

driver = Selenium::WebDriver.for :remote, url: "http://selenium:4444/wd/hub", desired_capabilities: :firefox

driver.navigate.to("http://example.com")
puts driver.title

driver.quit