[feature] Implement dotenv-expand to accept variable expansion in dot env files

Created by: andrewmclagan

Currently we use the dotenv library to parse a .env file in the project root and populate process.env. Great!

Although there exists scenarios in large scale deployments where the same environment variables are used accross many "app" or "services" or "containers"... For example

We have websites, API services and databases consuming a base set of environment variables such as:

  CDN
  STRIPE_KEY
  API_HOST
  VERSION_TAG

We don't want to have to add another set to this simply with the leading key REACT_APP_XXX. Dotenv has a has a sister project / plugin called dotenv-expand that allows, wait for it.... variable expansion in the file! e.g.

REACT_APP_CDN=${CDN}
REACT_APP_STRIPE_KEY=${STRIPE_KEY}
REACT_APP_API_HOST=${API_HOST}
REACT_APP_VERSION_TAG=${VERSION_TAG}