build: add experimental support for print flag

Print flag can be used to make additional information
requests about the build and print their results.

Currently Dockerfile supports: outline, targets, subrequests.describe

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

commands/print.go

@@ -0,0 +1,35 @@
+package commands
+
+import (
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/moby/buildkit/frontend/subrequests"
+	"github.com/moby/buildkit/frontend/subrequests/outline"
+	"github.com/moby/buildkit/frontend/subrequests/targets"
+)
+
+func printResult(f string, res map[string]string) error {
+	switch f {
+	case "outline":
+		if err := outline.PrintOutline([]byte(res["result.json"]), os.Stdout); err != nil {
+			return err
+		}
+	case "targets":
+		if err := targets.PrintTargets([]byte(res["result.json"]), os.Stdout); err != nil {
+			return err
+		}
+	case "subrequests.describe":
+		if err := subrequests.PrintDescribe([]byte(res["result.json"]), os.Stdout); err != nil {
+			return err
+		}
+	default:
+		if dt, ok := res["result.txt"]; ok {
+			fmt.Print(dt)
+		} else {
+			log.Printf("%s %+v", f, res)
+		}
+	}
+	return nil
+}