ensure mount in commit

builder.go

diff --git a/builder.go b/builder.go
index ab07233..420370b 100644
--- a/builder.go
+++ b/builder.go
@@ -124,6 +124,10 @@
 func (builder *Builder) Commit(container *Container, repository, tag, comment, author string, config *Config) (*Image, error) {
 	// FIXME: freeze the container before copying it to avoid data corruption?
 	// FIXME: this shouldn't be in commands.
+	if err := container.EnsureMounted(); err != nil {
+		return nil, err
+	}
+
 	rwTar, err := container.ExportRw()
 	if err != nil {
 		return nil, err