Update Dockerfile in j.podman.md

[bevzzz]

podman no longer supports docker.io as a default registry, so the Dockerfile should mention it explicitly.

The old behaviour can be restored by adding a config to /etc/containers/registries.conf:

unqualified-search-registries = ["docker.io"]

but I believe that specifying a fully-qualified name is a more pragmatic way to go.

The solution comes from this SE post.
Also see the related documentation for containers-registries.conf