A Jenkins agent using SSH to establish connection.
See Jenkins Distributed builds for more info.
To run a Docker container
docker run jenkinsci/ssh-slave "<public key>"
You'll then be able to connect this agent using ssh-slaves-plugin as "jenkins" with the matching private key.
To use this image with Docker Plugin, you need to
pass the public SSH key using environment variable JENKINS_SLAVE_SSH_PUBKEY
and not as a startup argument.
In Environment field of the Docker Template (advanced section), just add:
JENKINS_SLAVE_SSH_PUBKEY=<YOUR PUBLIC SSH KEY HERE>
Don't put quotes around the public key. You should be all set.