Make sure Jenkins user can access Docker group

Jenkins needs to be able to access the Docker socket
to start containers using the "Docker Cloud" plugin.

Change-Id: I59e8df8e7069ac79a92b7416ef3e73534b751514
diff --git a/jenkins-docker/master/jenkins.sh b/jenkins-docker/master/jenkins.sh
index a0536ba..a0d71b7 100755
--- a/jenkins-docker/master/jenkins.sh
+++ b/jenkins-docker/master/jenkins.sh
@@ -4,4 +4,5 @@
 rm -Rf /var/jenkins_home/nodes
 
 chown -R jenkins:dockergroup /var/jenkins_home
+usermod -G docker jenkins
 gosu jenkins /usr/local/bin/run-jenkins.sh $*