add jclouds-docker client reference

Docker-DCO-1.1-Signed-off-by: Andrea Turli <andrea.turli@gmail.com> (github: andreaturli)

Docker-DCO-1.1-Signed-off-by: Andrea Turli <andrea.turli@gmail.com> (github: )
Merge pull request #6870 from andreaturli/doc/jclouds-client

add jclouds-docker client reference