Hilariously, your post is now the top Google search result for
shell inside docker container unraid
and the link you provided is dead.
Since I can't easily find another good reference, I'll just post the solution here (for Unraid 6.11.5):
1. Open a terminal on your Unraid Host (you can use the web terminal feature if you like)
2. Run
docker ps
and note the ID of the container you wish to run.
3. Run
docker exec -it previously_noted_ID /bin/bash
(replace /bin/bash with a shell present in your container as necessary)