Add information about production considerations to docker-compose install guide.

[imbriaco]

In the previous docs, we had a callout that talked about persistent data to an extent:

For real use of Cog in a Docker environment (i.e., not using the example file directly), you'll want to give consideration to how you want to manage the data created in the containers, particularly the contents of the Postgres database and the /home/operable/.cogctl configuration file that is created by the bootstrap process (see below), which contains administrator credentials for accessing the system.

We should add something similar to the new cog-book documentation that talks about the various types of data that a user may want to persist. Off the top of my head this will probably include Postgres data, the Cog data directory (for log files and service data), Relay logs, and .cogctl.