Add guardrails for users that pull from or push to Docker Hub via the invalid `docker.io` domain

[baronfel]

We should add some registry-specific checks (like have been done for Amazon Elastic Container Registry endpoints) that issue a warning

• if the docker.io URI is used as the ContainerRegistry (because it won't work), or
• the registry.hub.docker.com or registry-1.docker.io domains are used as the ContainerRegistry but we only have credentials for index.docker.io (because the user used a 'normal' docker login command, which defaults to this domain)

We should also consider seeing if we can use the credentials from index.docker.io in this case to make things 'just work' and/or automatically rewrite docker.io urls to registry-1.docker.io urls.

Originally posted by @baronfel in https://github.com/dotnet/sdk-container-builds/discussions/359#discussioncomment-5125638