Trigger download of chrome/chromedriver binaries in CI

When a new chrome or chromedriver version is available, the CI build
updates it. In fact, each of the 32 rspec processes will try to download
the new version and then run chromedriver. This leads to problems as the
chromedriver binary being run may still be downloaded by another rspec
process. This leads to the following error: "Text file busy".

This commit adds a step to the CI build that downloads the binaries
before the tests are run.

docker/ci/entrypoint.sh

@@ -69,6 +69,10 @@ setup_tests() {
 
 	# create test database "app" and dump schema because db/structure.sql is not checked in
 	execute_quiet "time bundle exec rails db:migrate db:schema:dump zeitwerk:check"
+
+	# pre-cache browsers and their drivers binaries
+	execute "$(bundle show selenium)/bin/linux/selenium-manager --browser chrome --debug"
+	execute "$(bundle show selenium)/bin/linux/selenium-manager --browser firefox --debug"
 }
 
 run_units() {