title: heroku failed at the build script but heroku local web is fine
body: <p>I followed instructions, pushed repo to heroku and I find it crashed. 
  I use heroku/nodejs as a buildpack. </p>
  
  <p>Procfile: </p>
  
  <pre><code>web: npm run build &amp;&amp; npm run heroku-server
  </code></pre>
  
  <p>Scripts:</p>
  
  <pre><code>  "scripts": {
      "heroku-server": "cross-env SERVER_PROD_PORT=$PORT SERVER_PROD_HOST=0.0.0.0 npm run server",
      "browser": "cross-env NODE_ENV=development WEBPACK_CONFIG=browser_dev webpack-dev-server --open",
      "build": "cross-env NODE_ENV=production WEBPACK_CONFIG=browser_prod,server_prod webpack --colors",
      "build-analyze": "cross-env BUNDLE_ANALYZER=1 NODE_ENV=production WEBPACK_CONFIG=browser_prod,server_prod webpack --colors",
      "build-browser": "cross-env NODE_ENV=production WEBPACK_CONFIG=browser_prod webpack --colors",
      "build-run": "npm run build &amp;&amp; npm run server",
      "build-static": "cross-env NODE_ENV=production WEBPACK_CONFIG=static webpack --colors",
      "build-static-run": "npm run build-static &amp;&amp; npm run static",
      "clean": "rimraf dist",
      "lint": "eslint .",
      "server": "node dist/server",
  </code></pre>
  
  <p>When I run scripts from Procfile or $ heroku local web everything is builing fine. But pushed repo is throwing the errors in logs:</p>
  
  <pre><code>2018-01-30T22:23:05.780537+00:00 heroku[web.1]: Starting process with command `npm run build &amp;&amp; npm run heroku-server`
  2018-01-30T22:23:07.502982+00:00 heroku[web.1]: State changed from starting to crashed
  2018-01-30T22:23:07.491558+00:00 heroku[web.1]: Process exited with status 1
  2018-01-30T22:23:07.292672+00:00 app[web.1]:
  2018-01-30T22:23:07.292693+00:00 app[web.1]: &gt; beers@2.10.0 build /app
  2018-01-30T22:23:07.292694+00:00 app[web.1]: &gt; cross-env NODE_ENV=production WEBPACK_CONFIG=browser_prod,server_prod webpack --colors
  2018-01-30T22:23:07.292695+00:00 app[web.1]:
  2018-01-30T22:23:07.421065+00:00 app[web.1]: events.js:137
  2018-01-30T22:23:07.421068+00:00 app[web.1]:       throw er; // Unhandled 'error' event
  2018-01-30T22:23:07.421070+00:00 app[web.1]:       ^
  2018-01-30T22:23:07.421071+00:00 app[web.1]:
  2018-01-30T22:23:07.421073+00:00 app[web.1]: Error: spawn webpack ENOENT
  2018-01-30T22:23:07.421074+00:00 app[web.1]:     at _errnoException (util.js:1003:13)
  2018-01-30T22:23:07.421076+00:00 app[web.1]:     at Process.ChildProcess._handle.onexit (internal/child_process.js:201:19)
  2018-01-30T22:23:07.421078+00:00 app[web.1]:     at onErrorNT (internal/child_process.js:389:16)
  2018-01-30T22:23:07.421080+00:00 app[web.1]:     at process._tickCallback (internal/process/next_tick.js:152:19)
  2018-01-30T22:23:07.421081+00:00 app[web.1]:     at Function.Module.runMain (module.js:703:11)
  2018-01-30T22:23:07.421083+00:00 app[web.1]:     at startup (bootstrap_node.js:193:16)
  2018-01-30T22:23:07.421085+00:00 app[web.1]:     at bootstrap_node.js:617:3
  2018-01-30T22:23:07.426513+00:00 app[web.1]: npm ERR! code ELIFECYCLE
  2018-01-30T22:23:07.426855+00:00 app[web.1]: npm ERR! errno 1
  2018-01-30T22:23:07.428042+00:00 app[web.1]: npm ERR! beers@2.10.0 build: `cross-env NODE_ENV=production WEBPACK_CONFIG=browser_prod,server_prod webpack --colors`
  </code></pre>
  
  <p>What am I missing?</p>