Describe more clearly the authorization required by services such as sregistry

[prudhomm]

To run hifimagnet, one needs access to private sregistry. The procedure should be explain.
There is currently a problem with the workflow, as the credentials are not properly passed when the user buys the product. For example if access to private data (eg private singularity images) then proper credentials should be given extra (eg to sregistry at cesga)

the documentation should be more clear about that