Invalidate environment variable file using hash

[danielemery]

Currently when you deploy a new version of a docker-react app the browser caches the window.env.js file.

There are many ways this could be solved but the proposal I'm going with is to include a hash of the file's contents in the filename.

This will mean the index.html script tag will need to be updated each time the environment variables are changed, so this should be done as part of the npx docker-react prep command on container start.

This means this overlaps a little with #7