Pass variables into a podTemplate loaded from shared library?

Using Jenkins
1

In a shared library I have this generic pipeline:

sharedlib/vars/agentLauncher.groovy

def launch(Closure body) {
  String podLabel = "jenkins-agent-${UUID.randomUUID().toString()}"
  String agentImage = 'my-jenkins-agent:latest'
  env.agentImage = agentImage
  podTemplate(label: podLabel, yaml: libraryResource('agentPodTemplate.yaml')) {    
    timestamps {
        node(podLabel) {
            container('my-jenkins-agent') {
              //env.agentImage = agentImage
              body.call()
            }
        }
      }
  }
}

where I am trying to pass the agentImage as a parameter to the podTemplate (located in the same sharedLib) :

sharedlib/resources/agentPodTemplate.yaml

apiVersion: v1
kind: Pod
spec:
  containers:
  - name: jnlp
    image: jenkins/inbound-agent:4.11-1
  - name: my-jenkins-agent
    image: "${env.agentImage}"
    #image: ${agentImage}    
    #image: ${agentImage}          
    #image: my-jenkins-agent:latest
    command: ['cat']
    resources:
      limits:
        cpu : 1
        memory : 4Gi
      requests:
        cpu: 200m
        memory: 1Gi
    tty: true

But when I run that from my pipeline in another repo:

Jenkinsfile

@Library(value="sharedlib", changelog=false) _
agentLauncher.launch() {
    stage('Checkout') {
        checkout scm
    }
    stage('build') {
      container('my-jenkins-agent') {
        sh 'whoami'  
      }
    }    
}

I get the error:

[Warning][jenkins/jenkins-agent-e9b3e42c-6920-44f4-9a38-caab96a520f8-148zq-rwg3b][Failed] Error: InvalidImageName
[Warning][jenkins/jenkins-agent-e9b3e42c-6920-44f4-9a38-caab96a520f8-148zq-rwg3b][InspectFailed] Failed to apply default image tag "${agentImage}": couldn't parse image reference "${agentImage}": invalid reference format: repository name must be lowercase

so the variable is clearly not expanded/resolved at all.

I have looked at:

Any suggestions on how to make this work?