Convox linked container gets no HOST env var

gistfile1.txt

With the following `docker-compose.yml`:

sonar:
  build: ./sonar
  environment:
    - SONAR_DB_URL
  ports:
    - 9000
  links:
    - sonar_db
sonar_db:
  image: convox/postgres
  ports:
    - 5432

Running `env` in the `sonar` container gives me:

SONAR_DB_USERNAME=postgres                                                                     │
SONAR_DB_SCHEME=postgres                                                                       │
SONAR_DB_URL=postgres://postgres:password@:5432                                                │
SONAR_DB_PORT=5432
SONAR_DB_PASSWORD=password
SONAR_DB_HOST=