Schema 1 (`application/vnd.docker.distribution.manifest.v1+prettyjws`) has been
officially deprecated since containerd v1.7 (PR 6884).
We have planned to remove the support for Schema 1 in containerd v2.0, but this
removal may still surprise some users.
So, in containerd v2.0 we will just disable it by default.
The support for Schema 1 can be still enabled by setting an environment variable
`CONTAINERD_ENABLE_DEPRECATED_PULL_SCHEMA_1_IMAGE=1`, however, this workaround
will be completely removed in containerd v2.1.
Schema 2 was introduced in Docker 1.10 (Feb 2016), so most users should
have been already using Schema 2 or OCI.
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
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