Mounting a primary volume as a subdirectory of a sidekick volume

Rancher 1.x
1

I have a stack with two services – primary and sidekick. The primary sidekick mounts volumes from the sidekick, but also wants to mount a volume off a subdirectory of a volume mounted from a sidekick. This isn’t working. Any idea how to achieve my goal?

# sidekick holds a static site accessible at http://example.org
# newvol holds content to be accessible at http://example.org/another
version: '2'

services:
    primary:
        labels:
            io.rancher.sidekicks: sidekick
            rgon.domain: example.org
            rgon.port: 80
            rgon.ssl: 'true'
            rgon.redirect: 'https'
        image: nginx
        volumes_from:
            - sidekick
        volumes:
            - newvol:/usr/share/nginx/html/another
    sidekick:
        labels:
            io.rancher.container.pull_image: always
        image: my-primary
        volumes:
            - /usr/share/nginx/html
volumes:
    newvol: {}