remove buildkit as dependency

This copies the github.com/moby/buildkit/util/appcontext
package as an internal package. The appcontext package from
BuildKit was the only remaining dependency on BuildKit, and
while we may need some of its functionality, the implementation
is not correct for how it's used in docker/cli (so would need
a rewrite).

Moving a copy of the code into the docker/cli (but as internal
package to prevent others from depending on it) is a first step
in that process, and removes the circular dependency between
BuildKit and the CLi.

We are only using these:

    tree vendor/github.com/moby/buildkit
    vendor/github.com/moby/buildkit
    ├── AUTHORS
    ├── LICENSE
    └── util
        └── appcontext
            ├── appcontext.go
            ├── appcontext_unix.go
            ├── appcontext_windows.go
            └── register.go

    3 directories, 6 files

Before this:

    go mod graph | grep ' github.com/docker/cli'
    github.com/moby/buildkit@v0.11.6 github.com/docker/cli@v23.0.0-rc.1+incompatible

After this:

    go mod graph | grep ' github.com/docker/cli'
    # (nothing)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cmd/docker/docker.go

@@ -13,8 +13,8 @@ import (
 	"github.com/docker/cli/cli/command/commands"
 	cliflags "github.com/docker/cli/cli/flags"
 	"github.com/docker/cli/cli/version"
+	"github.com/docker/cli/cmd/docker/internal/appcontext"
 	"github.com/docker/docker/api/types/versions"
-	"github.com/moby/buildkit/util/appcontext"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"github.com/spf13/cobra"

cmd/docker/internal/appcontext/appcontext.go

@@ -0,0 +1,48 @@
+package appcontext
+
+import (
+	"context"
+	"os"
+	"os/signal"
+	"sync"
+
+	"github.com/sirupsen/logrus"
+)
+
+var (
+	appContextCache context.Context
+	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 := context.Background()
+		for _, f := range inits {
+			ctx = f(ctx)
+		}
+
+		ctx, cancel := context.WithCancel(ctx)
+		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
+}

cmd/docker/internal/appcontext/appcontext_unix.go

@@ -0,0 +1,12 @@
+//go:build !windows
+// +build !windows
+
+package appcontext
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+var terminationSignals = []os.Signal{unix.SIGTERM, unix.SIGINT}

cmd/docker/internal/appcontext/appcontext_windows.go

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

cmd/docker/internal/appcontext/register.go

@@ -0,0 +1,14 @@
+package appcontext
+
+import (
+	"context"
+)
+
+type Initializer func(context.Context) context.Context
+
+var inits []Initializer
+
+// Register stores a new context initializer that runs on app context creation
+func Register(f Initializer) {
+	inits = append(inits, f)
+}