Skip redundant setup for /dev/ptmx when specified explicitly in the OCI spec.

Per the OCI spec, /dev/ptmx is always a symlink to /dev/pts/ptmx. As such, if
the OCI spec has an explicit entry for /dev/ptmx, runc shall ignore it.

This change ensures this is the case. A integration test was also added
(in tests/integration/dev.bats).

Signed-off-by: Cesar Talledo <ctalledo@nestybox.com>

libcontainer/rootfs_linux.go

@@ -19,6 +19,7 @@ import (
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/system"
+	"github.com/opencontainers/runc/libcontainer/utils"
 	libcontainerUtils "github.com/opencontainers/runc/libcontainer/utils"
 	"github.com/opencontainers/selinux/go-selinux/label"
 
@@ -589,6 +590,12 @@ func createDevices(config *configs.Config) error {
 	useBindMount := system.RunningInUserNS() || config.Namespaces.Contains(configs.NEWUSER)
 	oldMask := unix.Umask(0000)
 	for _, node := range config.Devices {
+
+		// The /dev/ptmx device is setup by setupPtmx()
+		if utils.CleanPath(node.Path) == "/dev/ptmx" {
+			continue
+		}
+
 		// containers running in a user namespace are not allowed to mknod
 		// devices so we can just bind mount it from the host.
 		if err := createDeviceNode(config.Rootfs, node, useBindMount); err != nil {