[JENKINS-58631] Document defaultContainer for declarative

Author: Vlatombe

README.md

@@ -494,6 +494,28 @@ pipeline {
 }
 ```
 
+Run steps within a container by default. Steps will be nested within an implicit `container(name) {...}` block instead
+of being executed in the jnlp container.
+
+```
+pipeline {
+  agent {
+    kubernetes {
+      defaultContainer 'maven'
+      yamlFile 'KubernetesPod.yaml'
+    }
+  }
+
+  stages {
+    stage('Run maven') {
+      steps {
+        sh 'mvn -version'
+      }
+    }
+  }
+}
+```
+
 Run the Pipeline or individual stage within a custom workspace - not required unless explicitly stated.
 
 ```

src/main/resources/org/csanchez/jenkins/plugins/kubernetes/pipeline/KubernetesDeclarativeAgent/config.jelly

@@ -6,6 +6,9 @@
   <f:entry field="namespace" title="${%Namespace}">
     <f:textbox/>
   </f:entry>
+  <f:entry field="defaultContainer" title="${%Default container}">
+    <f:textbox/>
+  </f:entry>
   <f:entry field="inheritFrom" title="${%Pod template to inherit from}">
     <f:select/>
   </f:entry>

src/main/resources/org/csanchez/jenkins/plugins/kubernetes/pipeline/KubernetesDeclarativeAgent/help-defaultContainer.html

@@ -0,0 +1,4 @@
+<p>Name of the default container steps will be run in.</p>
+
+<p>When using this option, all steps will be executed within an implicit <code>container(name) {...}</code> block instead
+    of the jnlp container.</p>