Missing os label in the image manifest

Was trying to run some etcd image today:

```
$ sudo rkt run --insecure-options=image docker://quay.io/coreos/etcd:latest
image: using image from file /home/yifan/gopher/src/github.com/coreos/rkt/build-rkt-1.11.0+git/target/bin/stage1-coreos.aci
image: using image from local store for url docker://quay.io/coreos/etcd:latest
run: error setting up stage0: missing os label in the image manifest
```
