Investigate use of Redis/Memcache to create a shared cache for the Rails ECS cluster

[briri]

Build out the CF definition for a shared cache that the new dmsp_rails_prototype app can use.

If this seems like a good route to take, we will need to:

• [ ] Update the dmsp_rails_prototype config to use the cache
• [ ] Update the dmsp_rails_prototype docker-compose file to host a Redis cache