title: Appengine - tmp/ filesytem error when uploading a file
body: <p>I have an issue with Appengine (GCP) and file upload through Multer...</p>
  
  <p>I'd like to store in my DB a base64 image (whatever the reason). To do so, I try to upload this image to <code>/tmp</code> (<a href="https://cloud.google.com/appengine/docs/standard/nodejs/runtime" rel="nofollow noreferrer">apparently writable</a>) and then encode it to base64, then store it into DB.</p>
  
  <p>Locally, it works like a charm, but once deployed, it doesn't: <code>Error: EROFS: read-only file system, mkdir '/srv/tmp'</code></p>
  
  <p>Do you have any clue? Below my simplified code.</p>
  
  <pre><code>'use strict'
  
  const express = require('express')
  const router = new express.Router()
  const multer = require('multer')
  const upload = multer({ dest: 'tmp/' })
  const fs = require('fs')
  
  // APP DEPENDENCIES
  const { saveHousing } = require('../models/housing.js')
  
  
  /////////////////////////////////////
  router.post('/', upload.single('upload'), async (req, res) =&gt; {
  
    var img = fs.readFileSync(req.file.path).toString('base64')
    const housing = { IMG: { MIME: req.file.mimetype, DATA: img } }
    await saveHousing(housing)
  
    res.redirect(`/app/dashboard`)
  })
  </code></pre>
  
  <p>I'd like to not use Google Cloud Storage to simplify things...</p>