Add CNI support to ctr run

This adds linux cni support to `ctr run` via a `--cni` flag.  This uses the
default configuration for CNI on `ctr` to configure the network namespace for a
container.

Signed-off-by: Michael Crosby <michael@thepasture.io>

cmd/ctr/commands/run/run_unix.go

@@ -68,6 +68,10 @@ var platformRunFlags = []cli.Flag{
 		Usage: "set the CFS cpu quota",
 		Value: 0.0,
 	},
+	cli.BoolFlag{
+		Name:  "cni",
+		Usage: "enable cni networking for the container",
+	},
 }
 
 // NewContainer creates a new container