Remove ctareg credentials for CTA container registry/API access
We have been using the initial user based authentication mechanism for years to pull and push our containers in CI.
On the CTA operations side this mechanism has been superseeded by gitlab TOKEN based authentication for registry access: this is much better as it removes the need for ctareg CERN managed service account for this use case and the vulnerabilities associated with central account.
We need read TOKENS for the registry and the API in CI: 2 different tokens should be used.
This will also allow us to define 1 temporary token to allow external access to our registry during the CI handson during EOS workshop.