Add deprecation note for non-compliant registries

[thaJeztah]

• relates to Add fallback for pull by tag moby/moby#41749 Add fallback for pull by tag
• relates to [docker.pkg.github.com] Docker pull is not working on 20.10 RC1 moby/moby#41687 docker.pkg.github.com Docker pull is not working on 20.10 RC1
• relates to cache manifests on pull moby/moby#41607 cache manifests on pull
• relates to docker stack deploy fails to pull images from docker.pkg.github.com registry moby/moby#40403 docker stack deploy fails to pull images from docker.pkg.github.com registry
• relates to Docker service update --image "could not accessed on a registry to record its digest" moby/moby#34153 Docker service update --image "could not accessed on a registry to record its digest"
• relates to containerd can't pull image from Github Docker Package Registry containerd/containerd#3291 containerd can't pull image from Github Docker Package Registry

Docker Engine v20.10 and up includes optimizations to verify if images in the
local image cache need updating before pulling, preventing the Docker Engine
from making unnecessary API requests. These optimizations require the container
image registry to conform to the Open Container Initiative Distribution Specification
(https://github.com/opencontainers/distribution-spec).

While most registries conform to the specification, we encountered some registries
to be non-compliant, resulting in docker pull to fail.

As a temporary solution, Docker Engine v20.10 includes a fallback mechanism to
allow docker pull to be functional when using a non-compliant registry. A
warning message is printed in this situation:

WARNING Failed to pull manifest by the resolved digest. This registry does not
        appear to conform to the distribution registry specification; falling back to
        pull by tag. This fallback is DEPRECATED, and will be removed in a future
        release.

The fallback is added to allow users to either migrate their images to a compliant
registry, or for these registries to become compliant.

Note that this fallback only addresses failures on docker pull. Other commands,
such as docker stack deploy, or pulling images with containerd will continue
to fail.

Given that other functionality is still broken with these registries, we consider
this fallback a temporary solution, and will remove the fallback in an upcoming
major release.

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[thaJeztah]

@chris-crone @silvin-lubecki ptal

[silvin-lubecki]

LGTM

[chris-crone]

LGTM