Reproduce issue jenkinsci#434.

src/test/groovy/com/lesfurets/jenkins/unit/declarative/TestVariableAssignment.groovy

@@ -0,0 +1,23 @@
+package com.lesfurets.jenkins.unit.declarative
+
+import org.junit.Before
+import org.junit.Test
+
+class TestVariableAssignment extends DeclarativePipelineTest {
+
+    @Before
+    @Override
+    void setUp() throws Exception {
+        scriptRoots += 'src/test/jenkins/jenkinsfiles'
+        super.setUp()
+    }
+
+    @Test
+    void test() {
+        runScript("VariableAssignment_Jenkinsfile")
+        assertJobStatusSuccess()
+        printCallStack()
+        assertCallStack().contains("docker:[image:docker-image, reuseNode:false, stages:[:], args:, alwaysPull:true, containerPerStageRoot:false, label:docker-label")
+        assertCallStack().contains("dockerImage=docker-image, dockerLabel=docker-label, dockerPull=true")
+    }
+}

src/test/jenkins/jenkinsfiles/VariableAssignment_Jenkinsfile

@@ -0,0 +1,25 @@
+pipeline {
+    stages {
+        stage('Set variables') {
+            steps {
+                script {
+                    dockerImage = 'docker-image'
+                    dockerLabel = 'docker-label'
+                    dockerPull = true
+                }
+            }
+        }
+        stage('Use variables') {
+            agent {
+                docker {
+                    image dockerImage
+                    label dockerLabel
+                    alwaysPull dockerPull
+                }
+            }
+            steps {
+                echo "dockerImage=${dockerImage}, dockerLabel=${dockerLabel}, dockerPull=${dockerPull}"
+            }
+        }
+    }
+}