Due to the recent EC2 SSH Security change in 1.50.2, https://plugins.jenkins.io/ec2/#securing-the-connection-to-unix-amis

The expectation is that the SSH connection supports use of StrictHostKeyChecking property with values off, accept-new, yes when connecting to a newly spawned node.

Unfortunately Debian 9's ssh client does not support "off" or "accept-new" - it only supports yes/no, which means this plugin breaks in the LTS container.

Workaround is to use jenkins/jenkins:lts-slim container which is based on Debian 10/Buster