stage('clean') {
 def branches = []
 // extract available branches from git
 sh (returnStdout: true, script: "ssh-agent bash -c 'ssh-add /var/lib/jenkins/.ssh/id_rsa &>/dev/null; git ls-remote --heads --refs ssh://git@myrepo/project.git' | cut -f 2")
  .eachLine { branches << it }
 // extract all containers (including stopped)
 def containers = []
 sh (returnStdout: true, script: "docker ps -a --format '{{.Names}}' --filter name=project")
  .trim()
  .eachLine { containers << it }
 println(containers) // <---- only prints first container
 println(branches) // <---- only prints first branch

 //stop containers for non existing branches...
 containers.each{ containername ->
  if(branches.findAll({branch -> branch.contains(containername)}).isEmpty()){
   println("trying to stop ${containername}")
   //sh ("docker stop ${containername} || true") // container might already be stopped
   println("removing ${containername}")
   //sh ("docker rm ${containername}")
  }
 }
}