Unclear how to add global variables and `image: ...`

[hansbogert]

Just wandered into the world of dhall less than a day ago, this is awesome stuff. I'm not sure how complete this repo is deemed to be, but how can I do the following:

---
image: tmaier/docker-compose


variables:
  COMPOSE_FILE: 'docker-compose.yml'

...

Thanks!

[hansbogert]

Perhaps it was a conscious choice not to implement globals, but perhaps #4 is of interest.

This was made as a 'first encounter' to Dhall, I've no idea if I adhered to Dhall idioms.