Docker caches images aggressively, but it can cache problematic images. Try to disable it, especially on your CI server (when run time might be less of an issue):
<cache>false</cache>
Or temporarily using:
mvn <goals> -Ddocker.cache=false
You can see detailed JSON output using -X
mvn <goals> -X
docker:start can be started manually like this:
docker run -t -i -link example-project_mysql:db -P -p 8080:8080 $USER/example-project_app:1.0.0-SNAPSHOT
You can see the requests made to Docker using Wireshark. However, if you're using boot2docker, it'll be on a local loop-back interface.
It can be useful to see what's running, and watch will help you:
watch docker ps
You can tail the Docker logs, e.g.
boot2docker ssh
tail -f /var/log/docker.log