isn't this possible now with the yaml syntax?

The issue is because the JNLP agent container uses UID 10000 , in this example, the jenkins container uses the UID 1000 and both try to use the same user on the same folders with different UID

def label = "mypod-${UUID.randomUUID().toString()}"
def name = 'jenkins'
timestamps { 
  podTemplate(
    label: label,
    containers: [
      containerTemplate(name: name, image: "jenkins/jenkins", ttyEnabled: true)
    ]){
      node(label) {
        stage('Run on k8s'){
          sh 'id'
          container('jnlp') {
            sh 'id'
          }
          container(name) {
            sh 'id'
          }
        }
      }
    }
}

you can configure the user id using the yaml syntax

https://kubernetes.io/docs/tasks/configure-pod-container/security-context/#set-the-security-context-for-a-container

apiVersion: v1
kind: Pod
metadata:
  name: security-context-demo-2
spec:
  securityContext:
    runAsUser: 1000
  containers:
  - name: sec-ctx-demo-2
    image: gcr.io/google-samples/node-hello:1.0
    securityContext:
      runAsUser: 2000
      allowPrivilegeEscalation: false

The only solution it is to force the UID to 1000 and there is not conflict to access to the share filesystem

def label = "mypod-${UUID.randomUUID().toString()}"
def workspace = "/tmp/jenkins-${UUID.randomUUID().toString()}"
def name = 'jenkins'
def yaml = """
apiVersion: v1
kind: Pod
metadata:
  generateName: jnlp-
  labels:
    name: jnlp
    label: jnlp
spec:
  containers:
    - name: jnlp
    image: jenkins/jnlp-slave
    tty: true
    securityContext:
      runAsUser: 1000
      allowPrivilegeEscalation: false
    - name: jenkins
      image: jenkins/jenkins
      tty: true
      securityContext:
       runAsUser: 1000
       allowPrivilegeEscalation: false
"""
timestamps { 
  podTemplate(label: label, yaml: yaml){
    node(label) {
      sh 'id'
      stage('Run on k8s'){
        container('jnlp') {
          sh 'id'
        }
        container(name) {
          sh 'id'
        }
      }
    }
  }
}

Why is this resolved? Maybe not a bug but definitely sounds like a feature request. Jenkins official slave docker image use jenkins user with uid 10000, where most of the alpine based images on the market use root with uid 1000, which means anything created in Jenkinsfile by directives such as `writeFile` becomes unusable by other containers in the pod. Sounds pretty serious to me.

It's good to have an ability to fall back to the yml and have a workaround there, but more fundamental and transparent to the users solution is a must.

Support passing a user/uid into containerTemplate => it is supported using yaml syntax, will not add new features to containerTemplate

Feel free to open a PR

I agree with other users it would be quite helpful to add support through containerTemplate. 

Thanks, any update when can we expect the fix !!

Ohhh so cool to see it's in progress

Guys can you please approve [JENKINS-47827 ] adding support Support passing a user/uid into containerTemplate 
 

Waiting for code review and merge no conflicts and test covered and pass 

Guys, should I close it and close the PR? I see that there is a lot of people that resist this change adding this functionality, I have added it locally in our Jenkins because I have no time for endless conversions  

Oh no, what a shame really... Waiting for this feature since long time.

jonesbusy i finished adding it , only test not finished yet , but as i understand i got blocked