unix domain socket is not writable with vboxsf

write_sock.sh

# boot2docker ssh
# cd /Users/yoshida # mounted volume

# Can write text file into the vboxsf volume
docker@boot2docker:/Users/yoshida$ docker run -it --rm -v `pwd`:/tmp  ruby:2.0 touch /tmp/foo && ls foo
foo

docker@boot2docker:/Users/yoshida$ rm ./foo

# Cannot write sock file into the vboxsf volume
docker@boot2docker:/Users/yoshida$ docker run -it --rm -v `pwd`:/tmp  ruby:2.0 ruby -rsocket -e 's = UNIXServer.new("/tmp/foo")'
-e:1:in `initialize': Operation not permitted - "/tmp/foo" (Errno::EPERM)
	from -e:1:in `new'
	from -e:1:in `<main>'