Resolve the next.config.js up the tree to avoid it not being loaded when a source dir was specified via the CLI - solves vercel#3379 (vercel#3380)

Author: Inkdpixels

package.json

@@ -70,6 +70,7 @@
     "cross-spawn": "5.1.0",
     "del": "3.0.0",
     "etag": "1.8.1",
+    "find-up": "2.1.0",
     "fresh": "0.5.2",
     "friendly-errors-webpack-plugin": "1.6.1",
     "glob": "7.1.2",

server/config.js

@@ -1,5 +1,4 @@
-import { join } from 'path'
-import { existsSync } from 'fs'
+import findUp from 'find-up'
 
 const cache = new Map()
 
@@ -26,12 +25,13 @@ function loadConfig (dir, customConfig) {
     customConfig.configOrigin = 'server'
     return withDefaults(customConfig)
   }
-  const path = join(dir, 'next.config.js')
+  const path = findUp.sync('next.config.js', {
+    cwd: dir
+  })
 
   let userConfig = {}
 
-  const userHasConfig = existsSync(path)
-  if (userHasConfig) {
+  if (path && path.length) {
     const userConfigModule = require(path)
     userConfig = userConfigModule.default || userConfigModule
     userConfig.configOrigin = 'next.config.js'