Make app directory configurable via env variable

Created by: lordoffreaks

Given this type of structure, where cra lives in client

├── client
│   ├── README.md
│   ├── public
│   ├── src
│   ├── test
├── docker-compose.yml
├── package.json
├── server
│   ├── src
│   └── test
└── shared
    └── src
    └── test

Running APP_DIRECTORY=./client yarn start would make the app start, build and be tested with no issues

Obvisouly this is a POC and will need documentation and tests, but before going that far I'd like to check the opinion on the approach

Thanks