Fix container commit with podman

[cgwalters]

Regression from #367

Basically we only added a CI test with docker, not podman. And it turns out e.g.:

$ podman run --rm -ti quay.io/coreos-assembler/fcos:testing-devel
findmnt
TARGET                  SOURCE               FSTYPE   OPTIONS
/                       overlay              overlay  rw,relatime,context="system_u:object_r:container_file_t:s0:c64,c977",lowerdir=l/4CBPNAYHPZVNIEF3LWLSFMNBLU:l/YXA7RU6QD44
...
|-/run/.containerenv    tmpfs[/containers/overlay-containers/2bbbc4c081eb2a042a6aa7e12c57ebf60ee6593041d13878a539715671e55dac/userdata/.containerenv]
|                                            tmpfs    rw,nosuid,nodev,relatime,seclabel,size=6550144k,nr_inodes=1637536,mode=700,uid=1000,gid=1000,inode64
|-/run/secrets          tmpfs[/containers/overlay-containers/2bbbc4c081eb2a042a6aa7e12c57ebf60ee6593041d13878a539715671e55dac/userdata/run/secrets]

We need to change our handling for /run and /var cleanup to not try to traverse mounts.

[cgwalters]

Fixed by #377