Fix missing import

This import got lost after commit 56cc56b0fa
was merged, likely because the PR was built against an outdated
master.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

container/container_unix.go

@@ -18,6 +18,7 @@ import (
 	"github.com/docker/docker/volume"
 	volumemounts "github.com/docker/docker/volume/mounts"
 	"github.com/opencontainers/selinux/go-selinux/label"
+	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )