Automatically inject REACT_APP_ env vars into container

This change makes it easier to pass through REACT_APP_ variables into the container. Before this change, you would need to make a change in the CI to add the new variable and append it to the .env file. Now, you just need to have it set in your repo's CI variables and it will automatically get passed into the .env file.

This is already being used in builderqube, have a look for a working example.