Take proper jnlp image as default for Kubernetes execution (#759)

* Take proper jnlp image as default for Kubernetes execution

Following changes are contained:

* removal of custom jnlp image as default
* allow customization of jnlp image via system environment

fixes #757

* add documentation

resources/default_pipeline_environment.yml

@@ -38,7 +38,6 @@ general:
   githubServerUrl: 'https://github.com'
   gitSshKeyCredentialsId: '' #needed to allow sshagent to run with local ssh key
   jenkinsKubernetes:
-    jnlpAgent: 's4sdk/jenkins-agent-k8s:latest'
     securityContext:
     # Setting security context globally is currently not working with jaas
     #  runAsUser: 1000

test/groovy/DockerExecuteOnKubernetesTest.groovy

@@ -261,12 +261,10 @@ class DockerExecuteOnKubernetesTest extends BasePiperTest {
         assertThat(containerName, is('mavenexecute'))
 
         assertThat(containersList, allOf(
-            hasItem('jnlp'),
             hasItem('mavenexecute'),
             hasItem('selenium'),
         ))
         assertThat(imageList, allOf(
-            hasItem('s4sdk/jenkins-agent-k8s:latest'),
             hasItem('maven:3.5-jdk-8-alpine'),
             hasItem('selenium/standalone-chrome'),
         ))
@@ -388,6 +386,54 @@ class DockerExecuteOnKubernetesTest extends BasePiperTest {
         assertThat(podNodeSelector, is('size:big'))
     }
 
+    @Test
+    void testDockerExecuteOnKubernetesCustomJnlpViaEnv() {
+
+        nullScript.configuration = [
+            general: [jenkinsKubernetes: [jnlpAgent: 'config/jnlp:latest']]
+        ]
+        binding.variables.env.JENKINS_JNLP_IMAGE = 'env/jnlp:latest'
+        stepRule.step.dockerExecuteOnKubernetes(
+            script: nullScript,
+            juStabUtils: utils,
+            dockerImage: 'maven:3.5-jdk-8-alpine',
+        ) { bodyExecuted = true }
+        assertTrue(bodyExecuted)
+
+        assertThat(containersList, allOf(
+            hasItem('jnlp'),
+            hasItem('container-exec')
+        ))
+        assertThat(imageList, allOf(
+            hasItem('env/jnlp:latest'),
+            hasItem('maven:3.5-jdk-8-alpine'),
+        ))
+    }
+
+    @Test
+    void testDockerExecuteOnKubernetesCustomJnlpViaConfig() {
+
+        nullScript.configuration = [
+            general: [jenkinsKubernetes: [jnlpAgent: 'config/jnlp:latest']]
+        ]
+        binding.variables.env.JENKINS_JNLP_IMAGE = 'config/jnlp:latest'
+        stepRule.step.dockerExecuteOnKubernetes(
+            script: nullScript,
+            juStabUtils: utils,
+            dockerImage: 'maven:3.5-jdk-8-alpine',
+        ) { bodyExecuted = true }
+        assertTrue(bodyExecuted)
+
+        assertThat(containersList, allOf(
+            hasItem('jnlp'),
+            hasItem('container-exec')
+        ))
+        assertThat(imageList, allOf(
+            hasItem('config/jnlp:latest'),
+            hasItem('maven:3.5-jdk-8-alpine'),
+        ))
+    }
+
 
     private container(options, body) {
         containerName = options.name

vars/dockerExecuteOnKubernetes.groovy

@@ -106,6 +106,15 @@ import hudson.AbortException
 /**
  * Executes a closure inside a container in a kubernetes pod.
  * Proxy environment variables defined on the Jenkins machine are also available in the container.
+ *
+ * By default jnlp agent defined for kubernetes-plugin will be used (see https://github.com/jenkinsci/kubernetes-plugin#pipeline-support).
+ *
+ * It is possible to define a custom jnlp agent image by
+ *
+ * 1. Defining the jnlp image via environment variable JENKINS_JNLP_IMAGE in the Kubernetes landscape
+ * 2. Defining the image via config (`jenkinsKubernetes.jnlpAgent`)
+ *
+ * Option 1 will take precedence over option 2.
  */
 @GenerateDocumentation
 void call(Map parameters = [:], body) {
@@ -262,10 +271,17 @@ private void unstashWorkspace(config, prefix) {
 }
 
 private List getContainerList(config) {
-    def result = [[
-        name: 'jnlp',
-        image: config.jenkinsKubernetes.jnlpAgent
-    ]]
+
+    //If no custom jnlp agent provided as default jnlp agent (jenkins/jnlp-slave) as defined in the plugin, see https://github.com/jenkinsci/kubernetes-plugin#pipeline-support
+    def result = []
+
+    //allow definition of jnlp image via environment variable JENKINS_JNLP_IMAGE in the Kubernetes landscape or via config as fallback
+    if (env.JENKINS_JNLP_IMAGE || config.jenkinsKubernetes.jnlpAgent) {
+        result.push([
+            name: 'jnlp',
+            image: env.JENKINS_JNLP_IMAGE ?: config.jenkinsKubernetes.jnlpAgent
+        ])
+    }
     config.containerMap.each { imageName, containerName ->
         def containerPullImage = config.containerPullImageFlags?.get(imageName)
         def containerSpec = [