/proc/self/cgroup does not exist

Repro:
```
docker run --rm alpine cat /proc/self/cgroup
12:devices:/docker/75f4e1241b0fc88a27b062a41adbebded0ef4661d9ec1be45c4dfad120a1cf5d
...
```

```
docker run --rm --runtime=runsc alpine cat /proc/self/cgroup
cat: can't open '/proc/self/cgroup': No such file or directory
```

runsc version
```
runsc version release-20200127.0-132-ge07eacc99f00
spec: 1.0.1-dev
```

docker version
```
Client: Docker Engine - Community
Version: 19.03.3
API version: 1.40
Go version: go1.12.10
Git commit: a872fc2f86
Built: Tue Oct 8 00:59:59 2019
OS/Arch: linux/amd64
Experimental: false

Server: Docker Engine - Community
Engine:
Version: 19.03.3
API version: 1.40 (minimum version 1.12)
Go version: go1.12.10
Git commit: a872fc2f86
Built: Tue Oct 8 00:58:31 2019
OS/Arch: linux/amd64
Experimental: false
containerd:
Version: 1.2.6
GitCommit: 894b81a4b802e4eb2a91d1ce216b8817763c29fb
runc:
Version: 1.0.0-rc8
GitCommit: 425e105d5a03fabd737a126ad93d62a9eeede87f
docker-init:
Version: 0.18.0
GitCommit: fec3683
```

```
Linux tsundberg-dev 5.3.0-28-generic #30~18.04.1-Ubuntu SMP Fri Jan 17 06:14:09 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
```