cli/command/container: don't leak context

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

> cli/command/container/start.go:57::error: the cancelFun function is
> not used on all paths (possible context leak) (vet)
> cli/command/container/start.go:63::error: this return statement may be
> reached without using the cancelFun var defined on line 57 (vet)

> 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)

Do call the cancel function.

Note we might end up calling it twice which is fine as long as I can see
from the Go 1.10 source code.

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

cli/command/container/start.go

@@ -55,6 +55,7 @@ func NewStartCommand(dockerCli command.Cli) *cobra.Command {
 // nolint: gocyclo
 func runStart(dockerCli command.Cli, opts *startOptions) error {
 	ctx, cancelFun := context.WithCancel(context.Background())
+	defer cancelFun()
 
 	if opts.attach || opts.openStdin {
 		// We're going to attach to a container.