Add example for multi-environment Docker with a single image

[sparr]

Goals

1. Provide one or more example implementations for users who wish to deploy Next.js projects via Docker to multiple environments with a single container image

Non-Goals

1. Reproduce the existing one-image-per-environment example

Background

https://www.saltycrane.com/blog/2021/04/buildtime-vs-runtime-environment-variables-nextjs-docker/ describes various methods to use runtime configuration, and gives some snippets, but no fully working examples, and doesn't delve into the potential consequences of using methods like getServerSideProps

https://nextjs.org/docs/pages/building-your-application/configuring/environment-variables mentions the possibility of building an API to serve runtime configuration to otherwise-static pages, but no examples are given.

Proposal

Add a new example project that implements at least one, possibly more than one, method for providing runtime configuration for different environments, with the running code (and, thus, the contents of the container) not changing per environment.