url() references in css files do not use assetPrefix in transformed urls

This repo is a reproduction example of a assetPrefix bug that occurs in nextjs. References to files in css (i.e url()) are not using assetPrefix when they should.

Consider a font-face in a css file

@font-face {
  font-family: custom-font;
  src: url("./fonts/Itim-Regular.ttf");
}

This font url will be converted to /_next/static/media/... and will not use assetPrefix.

Run this repo to reproduce this example npm run build && npm run dev

Notice the font fails to load because it's url has no assetPrefix. All the other static assets are prefixed with assetPrefix