Add chromium-driver to Dockerfile

In the documentation about using docker, it says:

> System tests also work out of the box, although they might fail the
> first time while the tool running the tests downloads the right
> version of Chromedriver (which is needed to run them), and only
> "headless" mode (with a browser running in the background) is
> supported, which is the mode you'd probably use more than 95% of the
> time anyway.  For example, to run the tests for the homepage:
>
> POSTGRES_PASSWORD=password docker-compose run app bundle exec \
> rspec spec/system/welcome_spec.rb

For me, as predicted, the tests fail the first time, but they continue
to fail after. The errors are of form:

```
Failure/Error: example.run
Selenium::WebDriver::Error::WebDriverError:
  unable to connect to /home/consul/.cache/selenium/chromedriver/linux64
  /132.0.6834.110/chromedriver 127.0.0.1:9515
  # /usr/local/bundle/gems/selenium-webdriver-4.25.0/lib/selenium
  # /webdriver/common/service_manager.rb:132:in `connect_until_stable'
  # ... omitted ...
  # ./spec/spec_helper.rb:41:in `block (3 levels) in <top (required)>'
  # /usr/local/bundle/gems/i18n-1.14.6/lib/i18n.rb:353:in `with_locale'
  # ./spec/spec_helper.rb:40:in `block (2 levels) in <top (required)>'
```

Installing chromium-driver in the Dockerfile fixed it for me.

Dockerfile

@@ -18,7 +18,7 @@ RUN apt-get update -qq \
     unzip
 
 # Install Chromium for E2E integration tests
-RUN apt-get update -qq && apt-get install -y chromium
+RUN apt-get update -qq && apt-get install -y chromium chromium-driver
 
 # Files created inside the container repect the ownership
 RUN adduser --shell /bin/bash --disabled-password --gecos "" consul \