Inside we both know what's been going on

vars/retrieveEnvVars.groovy

@@ -0,0 +1,9 @@
+def call(String containerName, List<String> envVars) {
+    container(containerName) {
+        script {
+            envVars.each {  var ->
+                env."$var" = sh(script: "echo \$$var", returnStdout: true).trim()
+            }
+        }
+    }
+}