Split the domainname out of qualified hostnames.

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`.

Signed-off-by: Sam Hanes <sam@maltera.com>