Add chown to default nginx conf file, to make it owned by nginx user

[mattmatician]

Change owner of /etc/nginx/conf.d/default.conf to nginx:root

This change enables /etc/nginx/conf.d/default.conf changes through docker-entrypoint scripts.

/docker-entrypoint.d scripts are unable to write to /etc/nginx/conf.d/default.conf unless the owner of the file is changed to nginx.

The owner of /etc/nginx/conf.d/default.confis nginx:root in the nginxinc/nginx-unprivileged image.

Addressed Issue

Fixes #575

Checklist

• I have read and understand the contributing guidelines
• This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[nscuro]

Thanks @mattmatician!