Allow build/test hooks to receive environment variables

[jonpugh]

When running terra's hooks, I want users to be able to simply specificy things like:

hooks:
  build: | 
     drush site-install
  test: |
     cd tests && bin/behat

In order for these to work, we must pass the current URL and/or path to drush or behat, which is environment dependent.

I am currently assuming the hooks would be run on the docker host... perhaps we should switch that to run in the terrra/drush container? The drush container already has URI and ROOT set, so you don't need to specify those.

Now that I've typed this, I'm starting to think all the hooks should be run inside the container...

Thoughts?