This patch requests the OCI runtime to create a userns when the CRI
message includes such request.
This is an adaptation of a7adeb6976 ("cri: Support pods with user
namespaces") to sbserver, although the container_create.go parts were
already ported as part of 40be96efa9 ("Have separate spec builder for
each platform"),
Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>