Declarative: do not wrap steps with container statement unless default

container is changed

We met a weird issue when migrating the scripted syntax to declarative
with Kubernetes plugin: The `Launcher` doesn't forward `stdin`,
`stdout`, or `stderr` even though they are requested to be forwarded.
This breaks many plugins that use `Launcher` to start a remote process on Jenkins slaves.

This happens because when Kubernetes plugin converts declarative syntax to scripted syntax,
all build steps are enclosed with a `container` statement.
As a result, remote commands are run via the Kubernetes exec API rather than
the jnlp agent.

Since running steps via Kubernetes exec API is still considered ALPHA
and there are some compatibility issues to solve,
I would like to make a small change to the syntax converting process:
When the steps will not be enclosed with a `container` statement unless
an alternate default container is specified by the pipeline user.

src/main/resources/org/csanchez/jenkins/plugins/kubernetes/pipeline/KubernetesDeclarativeAgentScript.groovy

@@ -55,8 +55,14 @@ public class KubernetesDeclarativeAgentScript extends DeclarativeAgentScript<Kub
                             }
 
                             // call the main body
-                            script.container(container) {
+                            if (container == 'jnlp') {
+                                // If default container is not specified by the user,
+                                // do not wrap the body with `container` statement.
                                 body.call()
+                            } else {
+                                script.container(container) {
+                                    body.call()
+                                }
                             }
                         }.call()
                     }