Officially deprecate Schema 1

[AkihiroSuda]

Schema 1 has been substantially deprecated since circa. 2017 in favor of Schema 2 introduced in Docker 1.10 (Feb 2016) and its successor OCI Image Spec v1, but we have not officially deprecated Schema 1.

One of the reasons was that Quay did not support Schema 2 so far, but it is reported that Quay has been supporting Schema 2 since Feb 2020 (moby/buildkit#409).

This PR deprecates pulling Schema 1 images but the feature will not be removed before containerd 2.0.
Pushing Schema 1 images was never implemented in containerd (and its consumers such as BuildKit).

Docker/Moby already disabled pushing Schema 1 images in Docker 20.10 (moby/moby#41295), but Docker/Moby has not yet disabled pulling Schema 1 as containerd has not yet deprecated Schema 1. (moby/moby#42300 (comment))
Docker/Moby is expected to disable pulling Schema 1 images in future after this deprecation.

[thaJeztah]

LGTM

do you think we should/can print a deprecation message when someone is pulling a schema 1 image?

Oh! And is this "Schema 2 v1" or "Schema 1"? (I always confuse the two)

[fuweid]

LGTM

[estesp]

LGTM

[estesp]

@thaJeztah:

do you think we should/can print a deprecation message when someone is pulling a schema 1 image?

my only consideration here is that the most significant use of containerd is via other tools/layers for which we don't have direct access to stdout/err; so I think we could only realistically do this for ctr which is not a primary/recommended tool for users. I guess we could log CRI pulls as a daemon warning level message; again, not sure if that will really get the attention we would want from actual users.

Oh! And is this "Schema 2 v1" or "Schema 1"? (I always confuse the two)

This is the original schema 1 media type.

[dmcgowan]

I guess we could log CRI pulls as a daemon warning level message; again, not sure if that will really get the attention we would want from actual users.

For 1.7 I think we should add deprecation warning logs each time a deprecated feature is used. We don't need to do that in this PR, we should try to do them all together and consistently.