the combination --pod and --userns is already blocked. Ignore the
PODMAN_USERNS variable when a pod is used, since it would cause to
create a new user namespace for the container.
Ideally a container should be able to do that, but its user namespace
must be a child of the pod user namespace, not a sibling. Since
nested user namespaces are not allowed in the OCI runtime specs,
disallow this case, since the end result is just confusing for the
user.
Closes: https://github.com/containers/podman/issues/18580
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>