it is useful to migrate existing containers to a new version of
podman. Currently, it is needed to migrate rootless containers that
were created with podman <= 1.2 to a newer version which requires all
containers to be running in the same user namespace.
Closes: https://github.com/containers/libpod/issues/2935
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>