Docker container options conflict

[Legion2]

The winery docker image can't be used with volumes and WINERY_REPOSITORY_URL at the same time.

I used the latest https://github.com/OpenTOSCA/opentosca-docker master branch and added the following to the override file:

 winery:
    environment:
      WINERY_REPOSITORY_URL: https://github.com/OpenTOSCA/tosca-definitions-public.git
    volumes:
      - /var/opentosca/winery/repository:/var/opentosca/repository

This should enable the tosca-definitions-public repository and store it on the host path /var/opentosca/winery/repository so it can be modified.
But winery (docker CMD) fails: rm: cannot remove '/var/opentosca/repository': Device or resource busy, because it tries to delete the mounted directory.

Solutions:

1. don't allow both options at the same time and print a hint if someone tries to use both
2. don't try to delete the mounted directory, only delete the content in the directory

[miwurster]

Works with the latest master.