Allow to set environment variables to Nginx container

[fmenabe]

Pull Request

Description of the change

Add the nginx.extraEnv parameter allowing to pass environment variables to the Nginx container.

Benefits

This is for example useful to set the timezone of the Nginx container through TZ environment variable (cf #494 and https://github.com/nginxinc/docker-nginx/issues/149).

Possible drawbacks

None, the new parameter is optional.

Applicable issues

• fixes #494

Additional information

Checklist

• [X] DCO has been signed off on the commit.
• [X] Chart version bumped in Chart.yaml according to semver.
• [X] (optional) Variables are documented in the README.md