Extend docker compose example to single containers

[aszs]

base.yaml defines a DockerHost
type that lets use specify a list of docker-compose.yml files to deploy. See https://github.com/onecommons/unfurl-campsite for an example of it being used.

It would be nice to derive a type that just took a single container as an argument and generated a docker-compose file. The code this example was derived from (https://github.com/christippett/terraform-cloudinit-container-server) did just that but I removed that logic to make the example simpler. So we should move that logic to a derived type to keep things simple.