vendor: add buildkit

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/moby/buildkit/util/appcontext/appcontext.go

@@ -0,0 +1,41 @@
+package appcontext
+
+import (
+	"context"
+	"os"
+	"os/signal"
+	"sync"
+
+	"github.com/sirupsen/logrus"
+)
+
+var appContextCache context.Context
+var appContextOnce sync.Once
+
+// Context returns a static context that reacts to termination signals of the
+// running process. Useful in CLI tools.
+func Context() context.Context {
+	appContextOnce.Do(func() {
+		signals := make(chan os.Signal, 2048)
+		signal.Notify(signals, terminationSignals...)
+
+		const exitLimit = 3
+		retries := 0
+
+		ctx, cancel := context.WithCancel(context.Background())
+		appContextCache = ctx
+
+		go func() {
+			for {
+				<-signals
+				cancel()
+				retries++
+				if retries >= exitLimit {
+					logrus.Errorf("got %d SIGTERM/SIGINTs, forcing shutdown", retries)
+					os.Exit(1)
+				}
+			}
+		}()
+	})
+	return appContextCache
+}

vendor/github.com/moby/buildkit/util/appcontext/appcontext_unix.go

@@ -0,0 +1,11 @@
+// +build !windows
+
+package appcontext
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+var terminationSignals = []os.Signal{unix.SIGTERM, unix.SIGINT}

vendor/github.com/moby/buildkit/util/appcontext/appcontext_windows.go

@@ -0,0 +1,7 @@
+package appcontext
+
+import (
+	"os"
+)
+
+var terminationSignals = []os.Signal{os.Interrupt}