Docker: avoid removing of Ruby dependencies.

This closes #179 issue on GitHub.

pkg/docker/Dockerfile.perl5.24

@@ -73,7 +73,7 @@ RUN set -x \
 	&& apt-get install --no-install-recommends --no-install-suggests -y \
 						$unitPackages \
                         curl \
-	&& apt-get remove --purge --auto-remove -y apt-transport-https ca-certificates && rm -rf /var/lib/apt/lists/* /etc/apt/sources.list.d/unit.list \
+	&& apt-get remove --purge --auto-remove -y apt-transport-https && rm -rf /var/lib/apt/lists/* /etc/apt/sources.list.d/unit.list \
 	\
 # if we have leftovers from building, let's purge them (including extra, unnecessary build deps)
 	&& if [ -n "$tempDir" ]; then \