Update sylius 1.7 --> 1.8

Hello guys, I tried to update sylius to version 1.8. I look in this file https://github.com/Sylius/Sylius/blob/master/UPGRADE-1.8.md. I use docker. And I get this error

Have you an idea ?

Sylius 1.8 has not yet been released. See https://github.com/Sylius/Sylius/releases