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/client/connhelper/connhelper.go

@@ -0,0 +1,37 @@
+// Package connhelper provides helpers for connecting to a remote daemon host with custom logic.
+package connhelper
+
+import (
+	"context"
+	"net"
+	"net/url"
+)
+
+var helpers = map[string]func(*url.URL) (*ConnectionHelper, error){}
+
+// ConnectionHelper allows to connect to a remote host with custom stream provider binary.
+type ConnectionHelper struct {
+	// ContextDialer can be passed to grpc.WithContextDialer
+	ContextDialer func(ctx context.Context, addr string) (net.Conn, error)
+}
+
+// GetConnectionHelper returns BuildKit-specific connection helper for the given URL.
+// GetConnectionHelper returns nil without error when no helper is registered for the scheme.
+func GetConnectionHelper(daemonURL string) (*ConnectionHelper, error) {
+	u, err := url.Parse(daemonURL)
+	if err != nil {
+		return nil, err
+	}
+
+	fn, ok := helpers[u.Scheme]
+	if !ok {
+		return nil, nil
+	}
+
+	return fn(u)
+}
+
+// Register registers new connectionhelper for scheme
+func Register(scheme string, fn func(*url.URL) (*ConnectionHelper, error)) {
+	helpers[scheme] = fn
+}