expose buildId to custom webpack configs

readme.md

@@ -934,7 +934,7 @@ In order to extend our usage of `webpack`, you can define a function that extend
 // (But you could use ES2015 features supported by your Node.js version)
 
 module.exports = {
-  webpack: (config, { dev }) => {
+  webpack: (config, { buildId, dev }) => {
     // Perform customizations to webpack config
 
     // Important: return the modified config

server/build/webpack.js

@@ -335,7 +335,7 @@ export default async function createCompiler (dir, { buildId, dev = false, quiet
 
   if (config.webpack) {
     console.log(`> Using "webpack" config function defined in ${config.configOrigin}.`)
-    webpackConfig = await config.webpack(webpackConfig, { dev })
+    webpackConfig = await config.webpack(webpackConfig, { buildId, dev })
   }
   return webpack(webpackConfig)
 }