phantomjs broken in Docker image

[jantman]

1. To get phantomjs to even run (via ofxgetter), we need to add -e 'QT_QPA_PLATFORM=offscreen' to the docker exec command.
2. Once that's done, phantomjs dies with [FATAL] QFontDatabase: Cannot find font directory /usr/lib/x86_64-linux-gnu/fonts - is Qt installed correctly?. That can be fixed with ln -s /usr/share/fonts /usr/lib/x86_64-linux-gnu/fonts
3. But then phantomjs just crashes without any helpful information at all.

It looks like the jessie-backports phantomjs package is broken, at least on this Docker image, or else we need to upgrade the underlying images or OS. Might be better to try using the phantomjs install methods from one of these:

• https://github.com/mlaccetti/docker-debian-phantomjs/blob/master/Dockerfile
• https://github.com/josefcs/debian-phantomjs/blob/master/Dockerfile
• https://gist.github.com/Lockyy/1609879f0fe3ae682429

Also, should add a simple phantomjs test to the Docker tests.