Make `phab shell` go to the working dir of the app

[joachim-n]

Some projects using phab are set up in such a way that phab shell starts you in the working dir of the app, which is the root of the Composer project. This is useful because you can immediately run Drush commands, for instance.

On other projects, phab shell takes you to the root user's folder, which means every time you do it you have to remember to cd /var/www.

[mikran]

That rootFolder can be configured in host settings, see https://docs.phab.io/docker-integration.html#a-docker-configuration