Add deprecation note for non-compliant registries
Add deprecation note for non-compliant registries

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.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

docker/cli,thaJeztah/cli,thaJeztah/cli,cpuguy83/cli,cpuguy83/cli,docker/cli