Explicitly allow www-data access to /var/tmp/nginx
Explicitly allow www-data access to /var/tmp/nginx

In Alpine Linux, the default directory by used by nginx is
`/var/tmp/nginx`, compared to Ubuntu's `/var/lib/nginx`.

Currently, users cannot upload and download files due to permission
issues which is most likely caused by `/var/tmp/nginx` not having
correct permissions.

This change was not picked up when I migrated the image from Ubuntu to
Alpine and needs amending.

Alternatively, we could run the nginx process as the `nginx` user, but
that would need more changes which I would like to defer now.

ministryofjustice/cla_backend,ministryofjustice/cla_backend,ministryofjustice/cla_backend,ministryofjustice/cla_backend