`$IMAGE` is now set from `$APP` (app name)

Changes introduced in dokku (28de3ecaa3231a223f83fd8d03f373308673bc40)
now remove the need to specify *both* an app name and image name for
base dokku commands like: dokku release appname app/appname, which is
now: dokku release appname. Therefore, this change does not rely on
`$IMAGE` being passed to this plugin but rather sets it from the
appname (since by convention, `$IMAGE = 'app/appname'`).