Are you sure you’re its Iglu Server on http://localhost:8080? You mentioned Scala Stream Collector, but these are two completely different components. Asking because the URL looks completely valid Iglu Server endpoint.
Also, I’d recommend you to use Iglu Server 0.6.0 rather than 0.4.0 listed in snowplow-docker repo.
Just in case, there’s a new subcommand in igluctl 0.7.0 (which should be announced today):
$ igluctl server keygen --vendor-prefix com.snowplowanalytics http://localhost:8080/api 1d9c7e70-012b-11e8-ba89-0ed5f89f718b