integration/volume: Make escaping image symlink storage-independent

The image subpath test uses a fixed number of parent traversals to
reach an existing path outside the image root. User namespace
remapping adds another directory to the graph root, so the link
instead resolves to a nonexistent path and returns ErrNotAccessible.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

integration/volume/mount_test.go

@@ -325,7 +325,7 @@ func setupTestImage(t *testing.T, ctx context.Context, apiClient client.APIClien
 		FROM busybox as symlink
 		RUN mkdir /hack \
 			&& ln -s "../subdir" /hack/good \
-			&& ln -s "../../../../../docker" /hack/bad
+			&& ln -s ../../../../../ /hack/bad
 		#--
 		FROM scratch
 		COPY foo /