Give NPM registry option for downloading node tools (#2208)

(cherry picked from commit a5337ee)

gradle/node.gradle

@@ -18,6 +18,13 @@
 configure([project(":solr:packaging"), project(":solr:solr-ref-guide"), project(":solr:webapp")]) {
     apply plugin: "com.github.node-gradle.node"
 
+    def npmRegistry = "${ -> propertyOrEnvOrDefault("solr.npm.registry", "SOLR_NPM_REGISTRY", '') }"
+    if (!npmRegistry.isEmpty()) {
+        tasks.npmSetup {
+            args.addAll(['--registry', npmRegistry])
+        }
+    }
+
     ext {
         rootNodeDir = "$rootDir/.gradle/node"
         nodeProjectDir = file("$rootNodeDir/$project.name")

help/localSettings.txt

@@ -6,3 +6,15 @@ and save a project-local 'gradle.properties' file from a template,
 with some default settings which you can modify.
 
 See your 'gradle.properties' file for more details of those settings.
+
+Building without access to the internet
+---------------------------------------
+
+-- Node --
+There are a few tasks that rely on downloading node tools from NPM.
+These include the Reference Guide build, the integration (e2e) tests and the webapp build.
+Some of these can be disabled, however other tasks such as building the solr webapp often cannot be disabled.
+
+In order to set a NPM registry for Gradle to connect to, please use either:
+- The "SOLR_NPM_REGISTRY" environment variable
+- The "-Psolr.npm.registry" Gradle property.