daemon: fix docker cp when container source is /

Before 7a7357da, archive.TarResourceRebase was being used to copy files
and folders from the container. That function splits the source path
into a dirname + basename pair to support copying a file:
if you wanted to tar `dir/file` it would tar from `dir` the file `file`
(as part of the IncludedFiles option).

However, that path splitting logic was kept for folders as well, which
resulted in weird inputs to archive.TarWithOptions:
if you wanted to tar `dir1/dir2` it would tar from `dir1` the directory
`dir2` (as part of IncludedFiles option).

Although it was weird, it worked fine until we started chrooting into
the container rootfs when doing a `docker cp` with container source set
to `/` (cf 3029e765).

The fix is to only do the path splitting logic if the source is a file.

Unfortunately, 7a7357da added support for LCOW by duplicating some of
this subtle logic. Ideally we would need to do more refactoring of the
archive codebase to properly encapsulate these behaviors behind well-
documented APIs.

This fix does not do that. Instead, it fixes the issue inline.

Signed-off-by: Tibor Vass <tibor@docker.com>

daemon/archive.go

@@ -236,7 +236,13 @@ func (daemon *Daemon) containerArchivePath(container *container.Container, path
 	if driver.Base(resolvedPath) == "." {
 		resolvedPath += string(driver.Separator()) + "."
 	}
-	sourceDir, sourceBase := driver.Dir(resolvedPath), driver.Base(resolvedPath)
+
+	sourceDir := resolvedPath
+	sourceBase := "."
+
+	if stat.Mode&os.ModeDir == 0 { // not dir
+		sourceDir, sourceBase = driver.Split(resolvedPath)
+	}
 	opts := archive.TarResourceRebaseOpts(sourceBase, driver.Base(absPath))
 
 	data, err := archivePath(driver, sourceDir, opts, container.BaseFS.Path())
@@ -426,9 +432,6 @@ func (daemon *Daemon) containerCopy(container *container.Container, resource str
 		d, f := driver.Split(basePath)
 		basePath = d
 		filter = []string{f}
-	} else {
-		filter = []string{driver.Base(basePath)}
-		basePath = driver.Dir(basePath)
 	}
 	archive, err := archivePath(driver, basePath, &archive.TarOptions{
 		Compression:  archive.Uncompressed,