Added a feature to optionally override existing environment variables set by the system

[VarunAgw]

Fixes #1042

[nicolas-grekas]

Can you expand? Why can't you set your real env var to the value you need? Or why can't you remove said real env vars instead?
You can already override real env vars by listing them in the SYMFONY_DOTENV_VARS env var. Isn't this a better way to cover your use case?
Adding options adds complexity. Better not if possible.

[VarunAgw]

Hi @nicolas-grekas

Here is my code structure

.env

• Some data

.env.local

• APP_ENV=prod_conf1

.env.prod_conf1

• APP_ENV=prod
• APP_NAME=conf1
• some more data

.env.prod_conf2

• APP_ENV=prod
• APP_NAME=conf2
• some more data

Without an override flag, I can't override the value of APP_ENV in .env.prod_conf1

[VarunAgw]

I can't override APP_ENV using SYMFONY_DOTENV_VARS feature.