Take into consideration the environment configuration

[PGijsbers]

Following the convention from the dockerfile:

openml.org/docker/Dockerfile

Line 6 in 245d72c

RUN /app/server/src/client/app/node_modules/.bin/env-cmd -f /app/.env${environment} npm run build --prefix /app/server/src/client/app

I made modifications to the source code to similarly load a different .env file based on the environment defined.