Follow up with the previous link, if I want to run system test cases in Rails using:

class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  driven_by :selenium, using: :headless_firefox, screen_size: [1400, 1400]
end

Then I need to install Firefox, as well as fonts, so that the screenshots are rendered properly.

apk add xvfb firefox ttf-dejavu

(note: fonts are required for Firefox to render pages)