Fix possible systemd cgroups mangement bug

podman throws an error for all `RUN` commands saying:

```
systemd cgroup flag passed, but systemd support for managing cgroups is not available
```

This patch should fix this problem by changing the cgroups-manager
defaults to cgroupfs.

Dockerfile

 FROM fedora:30
 
-COPY resources/storage.conf /etc/containers/
+COPY resources/storage.conf resources/libpod.conf /etc/containers/
 
 RUN dnf -y upgrade && dnf -y install podman buildah findutils && dnf clean all
 

resources/libpod.conf

+cgroup_manager = "cgroupfs"