add support for domainname, qualified hostname

Docker doesn't like it when a fully qualified hostname is passed in the `hostname` parameter. When an FQDN is provided with `-h` the official CLI puts the first component in `hostname` and the rest in `domainname`. This change replicates that behavior when the user specifies an FQDN in `hostname` in their `fig.yml`. It also adds support for passing the `domainname` parameter directly.