sshagent must indeed be inside inside.

Not sure how to reproduce. It works for me.

Hmm, you're right. I am now unable to reproduce the issue I was having.

If you have complete, minimal, self-contained steps to reproduce the issue from scratch, please add them here and reopen.

Not sure offhand, would have to spend time digging into it. The obvious workaround is to just not use the SSH Agent plugin, and run ssh-agent yourself. You are probably better off not using Docker Pipeline either; just write a Dockerfile that sets things up the way you need it.

Seeing same, when using:

agent {
  docker {
    image 'myimage'
  }
}

I would see same as ihor_sviziev. Not for all commands though:

I think plain commands work:

sh "ssh user@remote.host.com rm -rf /"

but not when I am opening ssh tunnel and executing commands in the tunnel itself.

Also no issues with any commands (including more complex ones) when using agent { any } and executing directly on the Jenkins slave. So seems to be Docker-specific as well for me.

Any logging we can enable for the sshagent to see more details on where the issue is?

Well, I think I found how to reproduce the issue.

The following code works as expected if running on non-docker agent and does not work inside docker:

agent {
    docker {
        label 'bsp'
        image 'my-registry/my-image'
        reuseNode true
    }
}
stage("Some stage") {
    steps {
        sshagent(['some-id']) {
            script {
               sh "# some code that uses ssh"
            }
        }
    }
}

However, if I put sshagent() inside script {...} block it works:

agent {
    docker {
        label 'bsp'
        image 'my-registry/my-image'
        reuseNode true
    }
}
stage("Some stage") {
    steps {
        script {
            sshagent(['some-id']) {
               sh "# some code that uses ssh"
            }
        }
    }
}