Increase Dockerhub rate limit with authentication?

[adswa]

Recent Appveyor runs have failed due to dockerhub's rate limiting (see e.g., #227).
We had this in the early days of the datalad-dataverse extension, too, and a fix we temporarily employed was a Github action performing a DockerHub login (see datalad/datalad-dataverse@f6e8cfd)

I'm not sure if this is the right way to go about this, though. The past exploration was here: datalad/datalad-dataverse#47

[mih]

#199 is also a promising directory. If implemented, we would only pull each unique layer once for each CI run.

Better yet, we could preload the docker instance with all relevant images from an appveyor cache. Even for the actual image building, docker might be clever enough to reuse local image layers.

Caching and preloading is already done in the datalad-ria CI setup, and could be copied.