Codec rubydebug output not showing in 'docker logs' output

I am trying to debug the official Logstash Docker container. All available documentation I can find seems to imply that if I add an output stdout { codec => rubydebug }, I should be able to see this in docker logs. It's not working for me, however, although if I add --debug to the command line, I can see all that debug output fine.

Anyone have any clues on what I could be doing wrong?

In some more detail:

# docker pull
# docker run -v -d /path/to/logstash.conf:/etc/logstash/logstash.conf --name logstash logstash logstash -f /etc/logstash/logstash.conf

And in logstash.conf

output {
  stdout {
    codec => rubydebug

This was a newbie mistake. The root cause was not exposing the port to the Docker container, and the fix is adding a flag to the docker run command -p 6782:6782.