nginx-proxy/vhost-template.conf: Set X-Forwarded-Host

nginx-proxy/vhost-template.conf: Set X-Forwarded-Host

Since v1.5, Django cares about the requested Host (or
X-Forwarded-Host) [1], so we need to pass that information through.
With this Nginx config, you can set USE_X_FORWARDED_HOST [2] and parse
the appropriate hostname instead of your Docker container's IP.

[1]: https://docs.djangoproject.com/en/dev/ref/settings/#allowed-hosts
[2]: https://docs.djangoproject.com/en/dev/ref/settings/#std:setting-USE_X_FORWARDED_HOST