add merge volumes feature

Docker-DCO-1.1-Signed-off-by: cheneydeng <cheneydeng@qq.com> (github: cheneydeng)

the docker version now is not support that merge one source directory's contents with the target directory's contents in the container,you can only bind mount the source into the target directory while the target directory contents hidden with "bind" or mount the source directory in another 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