Allow BUILD_ID to be set in the environment

This makes multi server deploys with Capistrano possible.

server/build/index.js

@@ -8,7 +8,7 @@ import md5File from 'md5-file/promise'
 
 export default async function build (dir, conf = null) {
   const config = getConfig(dir, conf)
-  const buildId = uuid.v4()
+  const buildId = process.env.BUILD_ID || uuid.v4()
 
   try {
     await fs.access(dir, fs.constants.W_OK)