add merge volumes feature

Docker-DCO-1.1-Signed-off-by: cheneydeng <cheneydeng@qq.com> (github: cheneydeng)

the docker version now does not support to merge one source directory's contents with the target directory's contents in the container,we can only bind mount the source into the target while the target directory contents be hidden with "bind" or mount the whole volumes of another container in a fresh new directory in the container with "volumes-from".

this issue fixes this problem,it merges the source contents with the target contents.
here is the issue #3710