I found out recently that Modulus installs phantomjs@1.9.8 per default in the run image and the location is /usr/bin/phantomjs.
For people using phantom on Modulus (e.g. with dfischer:phantomjs) this might be interesting.
This also means that, if you want to use custom binaries (say a different version of phantom) you should prepend the location to PATH, not append. Otherwise, it will look in /usr/bin/ first.