Improve usage message; add aliases to the docker and chtc registries and set dockerhub as the default registry
Improve usage message; add aliases to the docker and chtc registries and set dockerhub as the default registry

(also they're called registries, not repositories)

htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor