docker provider: Don't explode when removing an image if it is in use
docker provider: Don't explode when removing an image if it is in use

Output from recent Docker has changed:

    Stderr: Error response from daemon: conflict: unable to delete 0ba49dd235e5 (cannot be forced) - image is being used by running container 250bbe980448

Fixes #7245

marxarelli/vagrant,sni/vagrant,marxarelli/vagrant,marxarelli/vagrant,mitchellh/vagrant,sni/vagrant,chrisroberts/vagrant,mitchellh/vagrant,marxarelli/vagrant,chrisroberts/vagrant,mitchellh/vagrant,mitchellh/vagrant,sni/vagrant,chrisroberts/vagrant,chrisroberts/vagrant,sni/vagrant