How to run multiple mongodb and mongos under docker?

Question

MongoDB

There is a desire to test how to work with mongos and several mongods.
ArchLinux system, kernel 3.12.7.
Installed docker 0.7.6. Found in the mongodb repository (dockerfile/mongodb).
I am running two mongods in different containers.
I'm not familiar with docker yet. I am only familiar with mongodb with mongod.
Questions:
1) how to modify the container to be able to run mongos?
2) how do I set up mongos (inside docker) to work with these mongods?