Add option to rspec:install generator to configure for Selenium in Docker #2878

[nicolasva]

issue : #2878

changes :
4 files modified/created:

1. [install_generator.rb]
   class_option :selenium_container (string, nil by default)
   Public method copy_system_test_configuration that generates the support file when the option is provided
   Private helper selenium_container
2. [system_test_configuration.rb]— ERB template that generates an RSpec.configure block with:
   served_by + remote driven_by when CAPYBARA_SERVER_PORT is set (Docker mode)
   Fallback to driven_by :selenium, using: :headless_chrome locally
3. [install_generator_spec.rb] — 7 new tests covering:
   File generation when option is provided
   Container hostname, served_by, remote driven_by, local fallback
   Custom hostname (e.g. --selenium-container=chrome)
   No file generated without the option
4. [Changelog.md] — New entry under the Development section

Usage:
rails generate rspec:install --selenium-container=selenium
Then uncomment the spec/support/**/*.rb glob in rails_helper.rb to auto-load the configuration.

[JonRowe]

Thanks for tackling this, I'd like to see some improvements to the specs but otherwise this is fine.