What does the first instruction in the "Instructions to run PDI Docker" page even mean?

[rduerr]

I have been running Docker on my iMac for several years now and the idea of logging into Docker seems irrelevant or at least it was not clear to me what that meant I should do...

[rduerr]

I've since learned that there are things called docker registries which one can log into. However, what registry? where?, is the issue. Clueless as to how to set up or install one (is one auto-enabled when docker starts?)...