Variables set in a stage are prematurely applied

[dblock]

Version report

1.12

Reproduction steps

stage('prepare') {
 agent any
 steps {
    script {
       dockerImage = 'docker-image'
    }
 }
}
stage('build') {
  agent {
    docker {
      label 'label'
      image dockerImage
  }
}

Results

Expected result:

image dockerImage should pickup the assigned docker-image label, which works in Jenkins.

Actual result:

Error that dockerImage is undefined.

[dblock]

I reproduced this in https://github.com/dblock/JenkinsPipelineUnit/tree/fix-434, but I couldn't fix it. Deferring evaluation of these variables looks involved. Could use help. /cc: @nre-ableton

com.lesfurets.jenkins.unit.declarative.TestVariableAssignment > test FAILED
    groovy.lang.MissingPropertyException: No such property: dockerImage for class: VariableAssignment_Jenkinsfile
        at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:66)

[nre-ableton]

Sorry -- I have no idea about any of the declarative API stuff.