Enable runtime configuration

[jake-low]

I've refactored the frontend Dockerfile and build process so that environment variables can now be injected into the app at container runtime (overriding any variables that were specified at build time e.g. in .env).

This will allow us to build a single image to run both in staging and production (using env vars to override the parts of the configuration that differs). That'll allow us to build images in CI, simplifying deployment both for us and for third parties who run their own MapRoulette instances.

Under the hood, here's how it works:

• Environment variables from .env are no longer injected into the JS code at build time. Instead, they're emitted into a separate env.json file.
• The Docker container dumps any environment variables beginning with REACT_APP_ into this env.json file when it starts, before launching the web server process. A variable specified like this at runtime overrides whatever value was given at compile time.
• The client JS code fetches /env.json and assigns it to window.env before bootstrapping the app. Application code uses window.env.VAR_NAME to access configuration variables (instead of process.env or import.meta.env). A preload hint is used to make sure that env.json is fetched early and doesn't delay the startup of the rest of the app more than necessary.

Ultimately this means you can docker run the frontend container with e.g. -e REACT_APP_MAP_ROULETTE_SERVER_URL to customize the URL that the app assumes it is running at, at runtime (without rebuilding the JS code or the container image). This allows the same container image to be built once and then deployed in different environments (staging, prod, third-party instances, etc).