This is an example/workable POC to get jenkins up and running with agents using a shared docker socket.
DOCKER_OPTS="-H unix:///var/run/docker.sock -H tcp://0.0.0.0:2375"
Must be in /etc/init/docker.conf
To run
docker-compose up
In Jenkins set the docker connection to tcp://HOSTNAME:2375 and use jenkins credentials for the jenkinscluster_jenkins_slave image to jenkins/jenkins