cpu-shares is a ‘relative weight’, relative to the default setting of 1024, so if you had two containers running on the same core, you could give them the CPU 50-50 or 80-20 or whatever you wanted by adjusting the numbers. It is an integer.
You cannot give an overall limit, as you want to, using this flag, but you can restrict the set of CPUs that the container runs on using --cpuset mentioned here.
The number 1024 is in the Cgroups docs.
This blog post from Marek Goldmann explains resource management in Docker.
See also Setting absolute limits on CPU for Docker containers, which says it can be done with lxc (older Docker implementation) but not libcontainer (current Docker implementation).