chore: add build-preview task for local build testing (#1539)

CONTRIBUTING.md

@@ -75,6 +75,16 @@ Runs the `clean` script and builds the CMS packages.
 yarn build
 ```
 
+### `build-preview`
+
+Runs the `build` and `build-preview` scripts in each package and serves the resulting build locally.
+
+#### Usage
+
+```sh
+yarn build-preview
+```
+
 ### `test`
 
 Runs all the CMS package tests.

package.json

@@ -4,6 +4,7 @@
     "start": "npm run bootstrap && npm run watch",
     "watch": "lerna run watch --parallel",
     "build": "npm run clean && lerna run build",
+    "build-preview": "npm run build && lerna run build-preview",
     "clean": "rimraf packages/*/dist",
     "reset": "npm run clean && lerna clean --yes",
     "test": "cross-env NODE_ENV=test jest --no-cache",

packages/netlify-cms/package.json

@@ -8,7 +8,8 @@
   "main": "dist/netlify-cms.js",
   "scripts": {
     "watch": "webpack-dev-server --hot --open",
-    "build": "cross-env NODE_ENV=production webpack"
+    "build": "cross-env NODE_ENV=production webpack",
+    "build-preview": "cross-env NODE_ENV=production webpack-dev-server --open"
   },
   "keywords": [
     "netlify",