docker-compose-dev.yml

version: "2"
services:
  app-rsync:
    volumes:
      - appcode-rsync-sync:/var/www:nocopy # will be mounted on /var/www

# that the important thing
volumes:
  appcode-rsync-sync:
external: true

docker-compose.yml

# This is our production compose file - not changed for docker-sync or similar

version: "2"
services:
  app-rsync:
    image: alpine
command: ['watch', '-n3', 'cat /var/www/index.html']

docker-sync.yml

version: "2"
options:
  verbose: true
syncs:
  #IMPORTANT: ensure this name is unique and does not match your other application container name
  appcode-rsync-sync: #tip: add -sync and you keep consistent names as a convention
    src: './app/'
    sync_host_ip: 'auto'
    sync_host_port: 10872
sync_strategy: 'rsync'