Update Docker Registry Location

[birm]

In https://github.com/mlpack/jenkins-conf/blob/master/REGISTRY_CONFIG_README.md

I don't think it's masterblaster.

[rcurtin]

So, I've actually disabled all the docker matrix builds because they're simply too expensive given the resources we now have available. I'm not sure if this is worth fixing then---I think we have good coverage through our Azure builds.

[birm]

Do you think it's worth it to remove or update the documents around the container registry?

[rcurtin]

Hmm, I think it's useful to keep the scripts that generated the Docker containers, but given that we don't currently have any registry set up, I don't have any issue removing or updating the documentation. What do you think? (Also sorry for the slow response!)