- Otherwise the docker-compose version is the latest available at time of build, which may not be compatible or expected
- The latest `COMPOSE_VERSION` can be taken by setting the argument blank (or leaving it as is in the case of `latest`)
- pip requires the `.*` to denote that we want the latest patch level, eg. just putting `docker-compose==1.11` installs `1.11.0` instead of `1.11.2`
- The extra `${COMPOSE_VERSION:+==}` is because pip errors out with `docker-compose==`
Fixes #4