Upgrade from Open Source to Enterprise Edition

[syed_ajwad]

I am using this Docker Image for Magento 2 given below and it is working perfect. I am using  CE 2.3.1

I want to upgrade to Enterprise Edition for that I have access keys. I tried editing composer.json and putting EE access keys in auth.json but mostly when I run

composer update

I get this error

[InvalidArgumentException]                                                        

  Could not find package magento/project-enterprise-edition with stability stable

Here are my efforts so far:
Upgrade by Setup Wizard

Upgrade by composer.json Edit

what is missing, how can it be done.