Add the FFI module

Problem/Motivation

The AI module requires the FFI PHP module for some of it's tests. We are currently compiling via a before_script, but it involves downloading php src (as the base image cleans it up), as well as a number of other dependencies. I think it'd be worth considering including in the build for the performance/maintenance overhead of module maintainers. I wouldn't expect it to make a drastic difference to the image size (though that could be validated).