Document a few image repositories, with some guidance for which to use

[choldgraf]

In #60 @yuvipanda added some helpful language about recent changes to DockerHub's terms of service, as well as pointers for alternative image repositories to use. There are probably others out there worth documenting as well, and it'd be better if we put comparisons between registries in a standalone section. I think that it'd be most-helpful if we did two things:

1. Provided instructions for a few (maybe the top 3-4?) image repositories.

   • AWS Elastic Container Registry
   • Google Container Registry (deprecated but popular)
   • Google Artifact Registry (preferred)
   • Azure Container Registry

2. Provided a short rubric for deciding between them (even if just a "we recommend this one for the simplest solution")

[yuvipanda]

We now have this for quay.io. I think we should have at least for the following:

1. AWS Elastic Container Registry
2. Google Container Registry (deprecated but popular)
3. Google Artifact Registry (preferred)
4. Azure Container Registry

Some of these will need #65 fixed as well

[sgibson91]

There's an Azure login GitHub Action for pushing to ACR that may help https://github.com/marketplace/actions/azure-container-registry-login

[choldgraf]

The AWS registry instructions are merged in (#78) and the next one we plan to prioritize is the Google registries. After that it might make sense to open a new issue specifically to track Azure.