Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/moby/buildkit/frontend/result.go

@@ -0,0 +1,25 @@
+package frontend
+
+import (
+	"github.com/moby/buildkit/solver"
+)
+
+type Result struct {
+	Ref      solver.ResultProxy
+	Refs     map[string]solver.ResultProxy
+	Metadata map[string][]byte
+}
+
+func (r *Result) EachRef(fn func(solver.ResultProxy) error) (err error) {
+	if r.Ref != nil {
+		err = fn(r.Ref)
+	}
+	for _, r := range r.Refs {
+		if r != nil {
+			if err1 := fn(r); err1 != nil && err == nil {
+				err = err1
+			}
+		}
+	}
+	return err
+}