Testing

cd demo
docker build -t demo .
docker run --privileged -v /sys/fs/cgroup:/sys/fs/cgroup:ro -p 127.0.0.1:22:22 -p 127.0.0.1:8080:8080 --rm --name demo demo &

As per this tip you then need to:

docker exec demo rm /var/run/nologin

Now to get a shell inside the container with a working X display (so you can interact with the Swing GUIs):

ssh -X jenkins@localhost xterm bash

The password is jenkins. You may need to first:

ssh-keygen -f ~/.ssh/known_hosts -R localhost

Prepare a Jenkins core build including the module and

docker cp …/jenkins.war demo:/tmp/jenkins.war

In the shell inside the container:

java -jar /tmp/jenkins.war &

From your host, to log in to http://localhost:8080/ use this password:

docker exec demo cat /home/jenkins/.jenkins/secrets/initialAdminPassword

Run through the setup wizard, installing no plugins. Configure security, enabling the JNLP agent port (random is fine). Manage nodes, create a permanent agent with the JNLP connector, using say /tmp as the home directory. Then in the container

curl -u admin:`cat ~/.jenkins/secrets/initialAdminPassword` http://localhost:8080/computer/agent/slave-agent.jnlp > /tmp/x.jnlp
javaws /tmp/x.jnlp

When prompted, install the agent as a service. Watch http://localhost:8080/computer/agent/log on host. From the container:

systemctl list-units jenkins-slave-*

to find the new service name. Then

sudo systemctl stop jenkins-slave-XXXXXXXX
sudo systemctl start jenkins-slave-XXXXXXXX
sudo systemctl restart jenkins-slave-XXXXXXXX

You should see it connecting and disconnecting.