Dockerfile: Work around an issue with phantomjs
Dockerfile: Work around an issue with phantomjs

When running the tests, cliver tries to check that PhantomJS's version
is within a certain range, by running "phantomjs --version".

The "phantomjs --version" command fails for some reason
on the new ruby:2.5.7-slim Docker base image.

Perhaps because the new Docker image is based on Debian 10 "Buster,"
whereas the old Docker image was based on Debian 9 "Stretch"?

This commit's workaround allows "phantomjs --version" to work again.

RefugeRestrooms/refugerestrooms,RefugeRestrooms/refugerestrooms,RefugeRestrooms/refugerestrooms,RefugeRestrooms/refugerestrooms