Schema 1 (`application/vnd.docker.distribution.manifest.v1+prettyjws`) has been
officially deprecated since containerd v1.7 (PR 6884), and disabled since v2.0 (PR 9765).
Users who have been seeing warnings like `conversion from schema 1 images is deprecated`
now have to rebuild the image with Schema 2 or OCI.
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>