If the docker container was started using /bin/bash (you can check using docker ps) command you can access it using attach.

docker attach container_name_or_id

If the docker container was started but not with /bin/bash, you have to create a bash instance inside the container using exec.

docker exec -it container_name_or_id /bin/bash 

Notice that attach does not create a new instance of bash but use the already created one in the corresponding docker container. However, both exec and run create new bash instances.

Examples

docker exec -it -u dclong jupyterlab-py jupyter nbconvert --to html --execute --ExecutePreprocessor.timeout=600 /workdir/spend.ipynb