Description of problem:
According to help document of atomic mount, the option --shared should be only supported by a container, but when you checked man page of atomic-mount, you will found as long as --live is not set, UUID or REPO refers to an image should be okay, in fact, I indeed can successfully mount an image w/ --shared option, however, it's a read-only mount permission, which can't be used by other images or containers.

Version-Release number of selected component (if applicable):

[cloud-user@atomic-host-001 atomic]$ sudo atomic host status
  TIMESTAMP (UTC)         VERSION     ID             OSNAME               REFSPEC                                                   
* 2016-06-18 15:21:12     7.2.5       9bfe1fb650     rhel-atomic-host     rhel-atomic-host-ostree:rhel-atomic-host/7/x86_64/standard

[cloud-user@atomic-host-001 atomic]$ rpm -q atomic docker
atomic-1.10.5-5.el7.x86_64
docker-1.10.3-44.el7.x86_64

[cloud-user@atomic-host-001 atomic]$ cat /etc/redhat-release 
Red Hat Enterprise Linux Atomic Host release 7.2

How reproducible:
always

Steps to Reproduce:
1. atomic mount -h
2. man atomic-mount     
3. docker pull busybox
4. atomic mount --shared <atomic_image_id> /mnt

Actual results:

[cloud-user@atomic-host-001 atomic]$ sudo docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

[cloud-user@atomic-host-001 atomic]$ sudo atomic images

  REPOSITORY          TAG      IMAGE ID       CREATED            VIRTUAL SIZE  
  docker.io/busybox   latest   sha256:2b8fd   2016-06-23 23:23   1.09 MB       

[cloud-user@atomic-host-001 atomic]$ sudo atomic mount --shared sha256:2b8fd /mnt

[cloud-user@atomic-host-001 atomic]$ sudo touch /mnt/tmp/test
touch: cannot touch ‘/mnt/tmp/test’: Read-only file system

[cloud-user@atomic-host-001 atomic]$ sudo grep mnt /proc/mounts 
/dev/mapper/docker-253:0-9197922-1bdd0508f7c1ecd5d49244fec5e355669697eb14a25068fda3f85affb354b384 /var/mnt xfs ro,context=system_u:object_r:usr_t:s0,nosuid,nodev,relatime,nouuid,attr2,inode64,sunit=1024,swidth=1024,noquota 0 0
/dev/mapper/docker-253:0-9197922-1bdd0508f7c1ecd5d49244fec5e355669697eb14a25068fda3f85affb354b384 /var/mnt xfs ro,context=system_u:object_r:usr_t:s0,nosuid,nodev,relatime,nouuid,attr2,inode64,sunit=1024,swidth=1024,noquota 0 0



# atomic mount -h
usage: atomic mount [-h] [-o OPTIONS] [--live | --shared] image mountpoint

positional arguments:
  image                 image/container id
  mountpoint            filesystem location to mount the image/container

optional arguments:
  -h, --help            show this help message and exit
  -o OPTIONS, --options OPTIONS
                        comma-separated list of mount options, defaults are
                        'ro,nodev,nosuid'
  --live                mount a running container 'live', allowing
                        modification of the contents.
  --shared              mount a container 'shared'. Mounts the container with
                        an SELinux label that other containers can read.

atomic mount attempts to mount a container image to a specified directory so
that its contents may be inspected.


# man atomic-mount| grep -A2 "image UUID"
       atomic  mount  attempts to mount the underlying filesystem of a container or image into the host filesystem. Accepts one of image UUID, container UUID, container NAME, or image REPO (optionally with
       registry and tag information).  If the given UUID or NAME is a container, and --live is not set, then atomic mount will create a snapshot of the container by committing it to a temporary  image  and
       spawning  a  temporary  container  from  that  image. If UUID or REPO refers to an image, then atomic mount will simply create a temporary container from the given image. All temporary artifacts are

Expected results:
only support --shared option for a container, and then it should be okay to fix document, if not, we should fix codes to support mount a shared image.

Additional info: