Add CDN support with assetPrefix

[arunoda]

Fixes #737

We should take this after #1611
(Then we need to do a rebase with master)

Sometimes, it's a pretty good idea to serve Next.js static assets via a CDN. You can easily do that by exposing the following option via next.config.js.

const isProd = process.NODE_ENV === 'production'
module.exports = {
  // You may only need to add assetPrefix in the production.
  assetPrefix: isProd? 'https://cdn.mydomain.com' : ''
}

Then inside your CDN configuration, set the origin URL as your app's domain name. That's all you've to.

With this setup, you can't add CDN support for static assets in the 'static' directory. This is only for core Next.js assets.

[stephenmathieson]

nbd, just curious: why did we move away from a function here?

[arunoda]

No any big reason. Do we really need it? Usually this is a build time config. I think it's better to have this a just a string.

[stephenmathieson]

no, I don't think we need it. #907 had it, so I figured we'd follow suit here. I suppose since distDir is a string, assetPrefix should be too tho.

[linus-amg]

it might be a function because we need to add the .json as well as rewriting / to /index, or does that happen automatically?

[linus-amg]

the function could receive 2 parameters, the route and the buildId

[linus-amg]

and returns the built url as a string, to get that resource fetched

[arunoda]

@elgubenis we don't wanna go that path.
We just wanna support typical CDNs and most of them as origin support and we don't want to re-write any URLs.

[stephenmathieson]

nit: "Page does not exist: ${page}"

[arunoda]

Thanks. This needs to go into no-eval branch. I'll do it.

[stephenmathieson]

@arunoda thanks for taking this on! I lost traction on my branch, so this is hugely appreciated. ❤️ ❤️

[arunoda]

This PR contains #1611 as well.

[sedubois]

Would there be a demo (e.g. with Now + CloudFlare) or some further doc which shows how to set this up? I haven't yet quite understood. Thanks a lot

[arunoda]

@sedubois see: https://www.maxcdn.com/one/tutorial/custom-cdn-integration/
In cloudflare, you don't need to do any of these. Just use cloudflare.

But this is for CDN where you only need to serve static content.