Multisite Support

[ccorda]

The current version of Bubs assumes all variables will be passed in as ENV variables. We've recently had a good deal of experience with using middleware hostname rewrites, and would like to support this first party in bubs-next.

A getSettings function that can either retrieve values from ENV vars, or using a hostname based lookup (and then passing those variables around) would allow us to have a common code for any calls that use these settings (client and server-side graphql), cache purging and other API routes, etc.

Also docs, docker.

Pieces of code that need to be handled:

• In getStaticPaths, we build each site. That needs to become an include so that we can custom per site and not have to update ..slug.js (see also Static generation paths #175)
• Our various api routes use getSettings, and check the headers. this needs to be abstracted, so that we can have preview.js for example be constant regardless of env vars or constants