add readme

README.md

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+# Jenkins SSH Agent with Podman
+
+Based on [jenkins/ssh-agent](https://hub.docker.com/r/jenkins/ssh-agent) with
+the addition of rootless podman.
+
+In order for running this container within podman, the following options should
+be set:
+
+- --security-opt=label=disable
+- --security-opt=unmask=ALL
+- --security-opt=seccomp=unconfined
+- --device=/dev/net/tun
+
+## Environment Variables
+
+- `JENKINS_AGENT_SSH_PUBKEY`: SSH public key accepted by the agent.
+
+## Volumes
+
+- `/home/jenkins`: state
+
+## Ports
+
+- `22`: ssh server