add support for --rootless

The --rootless flag was introduced in opencontainers/runc#1688.

In most cases runc itself can detect the appropriate value,
but it is considered to be there are some corner cases.

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

runc.go

@@ -66,6 +66,7 @@ type Runc struct {
 	Setpgid       bool
 	Criu          string
 	SystemdCgroup bool
+	Rootless      *bool // nil stands for "auto"
 }
 
 // List returns all containers created inside the provided runc root directory
@@ -655,6 +656,10 @@ func (r *Runc) args() (out []string) {
 	if r.SystemdCgroup {
 		out = append(out, "--systemd-cgroup")
 	}
+	if r.Rootless != nil {
+		// nil stands for "auto"
+		out = append(out, "--rootless", strconv.FormatBool(*r.Rootless))
+	}
 	return out
 }