Switch from x/net/context -> context

Since Go 1.7, context is a standard package. Since Go 1.9, everything
that is provided by "x/net/context" is a couple of type aliases to
types in "context".

Many vendored packages still use x/net/context, so vendor entry remains
for now.

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

client/container_restart_test.go

@@ -2,14 +2,13 @@ package client // import "github.com/docker/docker/client"
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"io/ioutil"
 	"net/http"
 	"strings"
 	"testing"
 	"time"
-
-	"golang.org/x/net/context"
 )
 
 func TestContainerRestartError(t *testing.T) {