This is the proposed fix for #2506. It provides a more complete message
with regards to name collisions including informing of the opposing
containers ID.
I have included a test to ensure that the correct short id is displayed
to make the message easier to understand.
In the dind environment running on a Fedora host, the lxc utils get
confused by the /sys/fs/cgroup/cpuacct,cpu cgroup mount and lxc-start
fails trying to access the wrong cgroup directory.
Each arg to docker run should be placed in a separate string.
Otherwise, when starting the command via exec.Cmd, the command is
interpreted as "echo test", which can't be found.
