This Jenkinsfile fragment fails:

```

docker.image("base/archlinux").inside {
    sh """

        sleep 10

        echo "Yay!"

    """
}

```

Since the `base/archlinux` Docker image (which you can find in the docker registry) doesn't have `ps`, durable-task fails to figure out the pid of the shell. What you see in Jenkins is the script runs for about a second, then is just aborted after Jenkins decides the process wasn't running after all. There's no helpful error explaining this situation, the script just... stops. It took quite a long time to figure out that the lack of `ps` was the problem.

Perhaps we can get it to check for `ps` (since it seems to be essential), and print a helpful error if it's not found?