Fix Docker Hub rate limit errors in CI

[lucaudill]

Over the last couple days, Docker Hub rate limit errors have started popping up in CI, making it fail. Apparently as of June 30, 2024, something has changed about the way Docker Hub rate limits work for Azure (perhaps due to some agreement ending on that date, see [1] and [2]). Regardless of the specifics, it looks like from now on CI is subject to the usual rate limit of 100 pulls per 6 hrs. We won't be able to get CI to pass reliably until we find a way around this rate limit. The first thing that comes to mind is for us to set up our own mirror registry that has the images we need for CI and just use that instead.

[1] - DockerHub Rate limit with Azure Container Instance and AKS
[2] - How to Avoid Docker Hub Rate Limits