Move drush cache inside /root/.cache

[grayside]

In order to further consolidate our build caches, we want all tools to use /root/.cache as the base directory. Drush should use /root/.cache/drush instead of /root/.drush.

This can be done by setting the CACHE_PREFIX environment variable like so:

ENV CACHE_PREFIX /root/.cache/drush

However, Drush does not take care of automatically creating this directory. We would need to specify that the volume used for caching is pre-created.

This would be really hard to handle automatically, unless we were to pursue #31 first.