PureFTPd is run within a docker container with volumes mounted from the host for access to content.
for additional details on volumes, ports etc refer to the container information docker inspect
You will manage users by entering the container (to be scripted)
docker exec -it pureftpd-server /bin/bash
NB: do not create scripts within the container if you don't know what you are doing, they will be lost on a rebuild. Instead consider creating scripts on the host that wrap around docker_exec} functionality