It's outdated, so don't use it.
- you can use just
redis
instead of nodebb/docker:ubuntu-redis and build nodebb image with Dockerfile
of NodeBB ( from official github repo ), but only for testing purpose.
- Or check mine ( NodeBB with Docker full chain ( nginx, redis, mongodb ) )

But here is genenal answers for your questions.
Basically If a container is alive ( means if you can see the container with docker ps
) then you connect to bash shell
docker exec -it <container name> bash
If a container is Exit
status, then it's more complicate. here An Golden Rule is that any modifications in containers is temporary and you will loose it. ( even there are ways to save a current container to image, but it's worst idea ).
Basically you can't connect to a container in Exit
status, but
- You can copy from or to the container with
docker copy
- You can manage files if you mounted
volumes
- After you solved problem then you can
docker restart
. BUT remember the Golden Rule
So best idea is, I think, you have to be able to make your own Dockerfile with proper options from scratch( means from ubunt
) or official docker image like redis
.
Docker is fun and powerful, fantastic, good to study, but much more complicate than without it before you have enough knowledge and skills. And even it will harm you easily.
Nevertheless, I don't discourage you to use docker!!, I encourage it 
I'm just giving a most important tip here. "Don't use docker for production before you know how to deal with a concept of a temporary container" but GOOD FOR TEST AND LEARN.
Welcome to docker 
And always do back them up. 