Short background:

If the container name is not "jnlp" the plugin will automatically insert a JNLP container, that pulls from docker hub. Naturally this does not work within a corporate network without internet connection.

If a private registry proxy is used it currently not possible to tell the Kubernetes plugin to use a different registry. 

Currently there are two REALLY bad workarounds:

• Start Jenkins with a property: org.csanchez.jenkins.plugins.kubernetes.PodTemplateBuilder.defaultImage (BAD: requires us to maintain image version using this property)
• Configure docker to use private docker registry by default. This is a big change, and not recommended. 

Suggestion: What about the option to prefix the default image with a custom docker registry? This could be an advanced UI option that when left alone does not change the behavior.

For instance: 

@VisibleForTesting
static final String DEFAULT_JNLP_IMAGE = System.getProperty(PodTemplateStepExecution.class.getName() + ".defaultImage", "jenkins/jnlp-slave:4.0.1-1");

What if we using like a UI option could prefix the fallback "jenkins/jnlp-slave:4.0.1-1" to "$registry/jenkins/jnlp-slave:4.0.1-1". That way, the plugin would maintain the version to be used, but be able to fetch from another registry.