Preserve environment variables from the parent environment

[pre]

It is useful to set runtime configuration with environment variables in Docker. Butterfly by default discards any environment variables set by the parent environment.

This PR copies env = os.environ and keeps them by passing --preserve-environment to /bin/su.

I did not test any other configurations than the one provided by --unsecure. This PR will probably break other configurations as-is. I am running Butterfly as a non-root user in Docker, without the ability to change the user. Access control is provided by a Nginx reverse proxy in the front.

[paradoxxxzero]

Err maybe a cli flag like --preserve-environment would be better to avoid breaking the current login functionality. And using the if branch just above instead of overriding the env var would be cleaner.

[CPapadim]

Any thoughts on adding this functionality?