container.runContainer(): call the cancel func

Switch from x/net/context to context made "go vet" see the previously
unseen error:

> cli/command/container/run.go:159::error: the cancelFun function is not
> used on all paths (possible context leak) (vet)
> cli/command/container/run.go:164::error: this return statement may be
> reached without using the cancelFun var defined on line 159 (vet)

As the cancelFun is only used for an attached container, use
context.WithCancel conditionally.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

cli/command/container/run.go

@@ -156,7 +156,12 @@ func runContainer(dockerCli command.Cli, opts *runOptions, copts *containerOptio
 		hostConfig.ConsoleSize[0], hostConfig.ConsoleSize[1] = dockerCli.Out().GetTtySize()
 	}
 
-	ctx, cancelFun := context.WithCancel(context.Background())
+	ctx := context.Background()
+	attach := config.AttachStdin || config.AttachStdout || config.AttachStderr
+	var cancelFun context.CancelFunc
+	if attach {
+		ctx, cancelFun = context.WithCancel(ctx)
+	}
 
 	createResponse, err := createContainer(ctx, dockerCli, containerConfig, &opts.createOptions)
 	if err != nil {
@@ -180,7 +185,6 @@ func runContainer(dockerCli command.Cli, opts *runOptions, copts *containerOptio
 			fmt.Fprintln(stdout, createResponse.ID)
 		}()
 	}
-	attach := config.AttachStdin || config.AttachStdout || config.AttachStderr
 	if attach {
 		if opts.detachKeys != "" {
 			dockerCli.ConfigFile().DetachKeys = opts.detachKeys