Add test for --net container:<id>

This adds an integration test for checking that the network namespace
fds are the same when a container joins another container's network
namespace.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

integration-cli/docker_cli_run_test.go

@@ -2638,3 +2638,37 @@ func TestRunModeIpcContainer(t *testing.T) {
 
 	logDone("run - hostname and several network modes")
 }
+
+func TestContainerNetworkMode(t *testing.T) {
+	cmd := exec.Command(dockerBinary, "run", "-d", "busybox", "top")
+	out, _, err := runCommandWithOutput(cmd)
+	if err != nil {
+		t.Fatal(err, out)
+	}
+	id := strings.TrimSpace(out)
+	if err := waitRun(id); err != nil {
+		t.Fatal(err)
+	}
+	pid1, err := inspectField(id, "State.Pid")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	parentContainerNet, err := os.Readlink(fmt.Sprintf("/proc/%s/ns/net", pid1))
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmd = exec.Command(dockerBinary, "run", fmt.Sprintf("--net=container:%s", id), "busybox", "readlink", "/proc/self/ns/net")
+	out2, _, err := runCommandWithOutput(cmd)
+	if err != nil {
+		t.Fatal(err, out2)
+	}
+
+	out2 = strings.Trim(out2, "\n")
+	if parentContainerNet != out2 {
+		t.Fatalf("NET different with --net=container:%s %s != %s\n", id, parentContainerNet, out2)
+	}
+	deleteAllContainers()
+
+	logDone("run - container shared network namespace")
+}