This is more in line with other consts that are used for defaults, and makes it
slightly easier to consume than DefaultV2Registry, e.g. see:
https://github.com/oras-project/oras-go/blob/v1.1.0/pkg/auth/docker/resolver.go#L81-L84
Note that both the "index.docker.io" and "registry-1.docker.io" domains
are here for historic reasons and backward-compatibility. These domains
are still supported by Docker Hub (and will continue to be supported), but
there are new domains already in use, and plans to consolidate all legacy
domains to new "canonical" domains. Once those domains are decided on, we
should update these consts (but making sure to preserve compatibility with
existing installs, clients, and user configuration).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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