When you do, say, `eris services start btcd` what is happening is that you are running a bitcoin full node using the (https://github.com/btcsuite/btcd) btcd program. By default it does not mine, but runs a full node. You can configure the node however you like according to the btcd's configuration.
Eris is doing no more that starting and stopping the node according to however you tell eris to start it in the btcd service definition file which should be located in ~/.eris/services/btcd.toml.
That was good fodder for a blog post. Hope you don't mind me answering there....
just read great answer! I imagined the file would be in a container but didn't know exactly where and how to work with it. thank you