Alright, found out a few things.
Went up to the 60 or so min. mark of the video.
Got the postgresql container id using
the most upvoted one directly below the one marked as an answer.
I'm not positive about the following, as I'm not overly familiar with networks or Docker, but I think the bridge is Docker's default network, and its gateway ip is how external services access containers, via any externally published ports. A container could also access a container this way, it seems, and it seems can also use the container ip along with the internal port.
Going to ask some colleagues who know more to comment on this, but you can work with this for now (I think to a large degree this isn't unique to Docker, but to networking and gateways in general), and if anyone else knows more please post as well!
update: Bridge's gateway ip was used, maybe in retrospect unnecessarily, because there was thought to be issues on OS X when connecting to another container. Can't get container name to work in connection string, but container's ip does work (see above), and that's going to be a bit quicker than going through the bridge's gateway ip.