mounts/validate: Don't check source exists with CreateMountpoint

Don't error out when mount source doesn't exist and mounts has
`CreateMountpoint` option enabled.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 05b883b)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

volume/mounts/linux_parser.go

@@ -94,7 +94,9 @@ func (p *linuxParser) validateMountConfigImpl(mnt *mount.Mount, validateBindSour
 			if err != nil {
 				return &errMountConfig{mnt, err}
 			}
-			if !exists {
+
+			createMountpoint := mnt.BindOptions != nil && mnt.BindOptions.CreateMountpoint
+			if !exists && !createMountpoint {
 				return &errMountConfig{mnt, errBindSourceDoesNotExist(mnt.Source)}
 			}
 		}