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.

Dockerfile

@@ -15,6 +15,9 @@ RUN curl -L https://bitbucket.org/ariya/phantomjs/downloads/phantomjs-2.1.1-linu
   # Clean up extra (un-needed) PhantomJS files
   && rm -rf phantomjs-2.1.1-linux-x86_64/
 
+# Work around an issue with running "phantomjs --version"
+ENV OPENSSL_CONF=/etc/ssl/
+
 # Specify a major version of Node.js to download and install
 ENV NODEJS_MAJOR_VERSION=10