This allows to solve the recurrent docker login issue by passing the
credentials to kubernetes as a secret.
Indeed docker login credentials are valid for 3 months then it is over.
We just need to pass the `ctareg` service account credentials in the
`ctaregsecrets` and we are done.
In addition `imagePullSecrets` works at the full cluster level as each
kubelet needs the registry credentials to pull its own image.
If `ctaregsecret` secret does not exists, kubernetes is perfectly fine
with that and uses the default authentication. This means that there is
no need to define this secret (which is useless in the buildtree
environment).