Defect: If Docker Pipeline plugin is not installed, terraform-pipeline cannot load

[kmanning]

• Docker should be optional - you can leverage a Dockerfile or docker container to run your pipeline, if you configure the AgentNodePlugin to do so.
• If you do not want to use Docker however, you should still be able to use terraform-pipeline. If you are not using Docker for terraform-pipeline, you should not need to install docker-related Jenkins plugins like Docker Pipeline.

Defect Present: v5.13, likely all the way back to v5.0
Steps to reproduce:

• Do not have Docker Pipeline plugin installed in your jenkins instance
• Load teraform-pipeline in a Jenkinsfile. Eg:

@Library(['terraform-pipeline@v5.13']) _

Jenkinsfile.init(this)

• Run your pipeline

Expected: The pipeline runs successfully, and does nothing
Actual: The pipeline fails with the error message:

hudson.remoting.ProxyException: groovy.lang.MissingPropertyException: No such property: docker for class: WorkflowScript
	at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:53)
	at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.getProperty(ScriptBytecodeAdapter.java:458)
	at com.cloudbees.groovy.cps.sandbox.DefaultInvoker.getProperty(DefaultInvoker.java:39)
	at com.cloudbees.groovy.cps.impl.PropertyAccessBlock.rawGet(PropertyAccessBlock.java:20)
	at Jenkinsfile.init(Jenkinsfile.groovy:59)