bake: allow overriding no-cache and pull per target via --set

Signed-off-by: Tibor Vass <tibor@docker.com>

util/flagutil/flagutil.go

@@ -0,0 +1,36 @@
+package flagutil
+
+import "strconv"
+
+type tristate struct {
+	opt *bool
+}
+
+// Tristate is a tri-state boolean flag type.
+// It can be set, but not unset.
+func Tristate(opt *bool) tristate {
+	return tristate{opt}
+}
+
+func (t tristate) Type() string {
+	return "tristate"
+}
+
+func (t tristate) String() string {
+	if t.opt == nil {
+		return "(unset)"
+	}
+	if *t.opt {
+		return "true"
+	}
+	return "false"
+}
+
+func (t tristate) Set(s string) error {
+	b, err := strconv.ParseBool(s)
+	if err != nil {
+		return err
+	}
+	t.opt = &b
+	return nil
+}