Need 'crossmnt' flag to allow volumes inside exported dir

[cuchac]

Hello,

to support docker volumes inside exported directory, there is necessary to add crossmnt flag.

Example docker-compose.yml

version: '3.3'

services:
 nfs:
  image: itsthenetwork/nfs-server-alpine:latest
  privileged: true
  volumes:
    - ./export:/export
    - type: volume
      source: somevolume
      target: /export/somevolume
  environment:
    - SHARED_DIRECTORY=/export

volumes:
  somevolume: ~

[colearendt]

Related to #23 , which allows setting options yourself more easily