Allow BUILD_ID to be set in the environment

This makes multi server deploys with Capistrano possible.

server/build/index.js

@@ -3,13 +3,13 @@ import fs from 'mz/fs'
 import uuid from 'uuid'
 import webpack from 'webpack'
 import getConfig from '../config'
-import {PHASE_PRODUCTION_BUILD} from '../../lib/constants'
+import { PHASE_PRODUCTION_BUILD } from '../../lib/constants'
 import getBaseWebpackConfig from './webpack'
 import md5File from 'md5-file/promise'
 
-export default async function build (dir, conf = null) {
+export default async function build(dir, conf = null) {
   const config = getConfig(PHASE_PRODUCTION_BUILD, dir, conf)
-  const buildId = uuid.v4()
+  const buildId = process.env.BUILD_ID || uuid.v4()
 
   try {
     await fs.access(dir, fs.constants.W_OK)
@@ -34,7 +34,7 @@ export default async function build (dir, conf = null) {
   }
 }
 
-function runCompiler (compiler) {
+function runCompiler(compiler) {
   return new Promise(async (resolve, reject) => {
     const webpackCompiler = await webpack(await compiler)
     webpackCompiler.run((err, stats) => {
@@ -54,7 +54,7 @@ function runCompiler (compiler) {
   })
 }
 
-async function writeBuildStats (dir, config) {
+async function writeBuildStats(dir, config) {
   // Here we can't use hashes in webpack chunks.
   // That's because the "app.js" is not tied to a chunk.
   // It's created by merging a few assets. (commons.js and main.js)
@@ -68,7 +68,7 @@ async function writeBuildStats (dir, config) {
   await fs.writeFile(buildStatsPath, JSON.stringify(assetHashMap), 'utf8')
 }
 
-async function writeBuildId (dir, buildId, config) {
+async function writeBuildId(dir, buildId, config) {
   const buildIdPath = join(dir, config.distDir, 'BUILD_ID')
   await fs.writeFile(buildIdPath, buildId, 'utf8')
 }