run: Pass configuration to test-pack as an environment variable

... rather than as a volume.  Volumes aren't quite the right fit here.
They would be useful if the config were to change while the test-pack
is running but that makes no sense.  Otherwise they complicate things
because all volumes are relative to the host root rather than the
service container root, and thus must be stored on a volume.

This also makes it easier to have ephermeral configurations.