error message for --push option

Signed-off-by: kotaroooo0 <k33asby@gmail.com>

build/build.go

@@ -421,7 +421,7 @@ func toSolveOpt(d driver.Driver, multiDriver bool, opt Options, dl dockerLoadCal
 			opt.Exports[i].Type = "moby"
 			if e.Attrs["push"] != "" {
 				if ok, _ := strconv.ParseBool(e.Attrs["push"]); ok {
-					return nil, nil, errors.Errorf("auto-push is currently not implemented for docker driver")
+					return nil, nil, errors.Errorf("builder instance is required to use auto-push")
 				}
 			}
 		}