We were installing .deb's from Debian. The next apt-get upgrade would cause roundcube to be upgraded.
Maybe that also triggered the installation of apache. Now install roundcube from Ubuntu. So long as
Ubuntu doesn't post an update to roundcube, at least it won't trigger an upgrade on the next
apt-get upgrade. This should also mean we don't need to purge the installation of apache2.

Also try using apt-mark hold to prevent roundcube from being updated, in case that will trigger
dependencies.