fix: #7326 map docker.io logout to the index server

[yin2hao]

fix #7326

docker login docker.io stores Docker Hub credentials under https://index.docker.io/v1/

Previously, docker logout docker.io treated docker.io as a custom registry and attempted to remove credentials using the literal hostname. As a result, the credentials stored for Docker Hub could remain in the credential store.

This change maps docker.io to https://index.docker.io/v1/, making docker logout docker.io behave consistently
with docker logout docker.io.

However, this change prevents credentials stored under the docker.io key in an existing config.json from being removed by this command. However, credentials are written under this key only when logging in with docker login https://docker.io, so the impact should be minimal.