Add /etc/nginx/conf.d to docker volumes for .htpasswd files

Useful when adding .htpasswd for basic auth. No other volume is appropriate for adding .htpasswd locations.

example:

```
  location / {
    proxy_pass http://es:9200;
    proxy_read_timeout 90;
    auth_basic "Restricted";
    auth_basic_user_file /etc/nginx/conf.d/elasticsearch.htpasswd;
  }
```