Enable the use of SCUBA_ROOT in .scuba.yml

[JonathonReinhart]

There are cases where it would be nice to set variables in .scuba.yml which are paths, based on SCUBA_ROOT . However, that is only accessible inside the container. (https://scuba.readthedocs.io/en/latest/environment.html)

cc @rpasek

[JonathonReinhart]

With #218, I think this is unnecessary.

• volumes
  • In a container path: SCUBA_ROOT is pointless, because the SCUBA_ROOT is already bind-mounted into the container as the primary purpose of Scuba.
  • In a host path: Now you can just use ./ which is relative to the scuba root.