Feature request: DIP_ROOT_DIR env var

[entr]

This is a feature request to introduce a new env variable DIP_ROOT_DIR which gives you the absolute location of the main dip.yml.

Why this is useful?

It opens up the possibility to create dip interactions that use the root directory while being executed from a subdirectory. Our specific use-case is that we have a number of projects in subdirectories and we want an interaction that can do something like cd "$DIP_ROOT_DIR/www/$*" && dip up -d to allow us to quickly move between projects.