Matrix
This guide tells me how to create users, since I've used a bit of a workaround to get it working. You can visit the chat over here (opens in a new tab)
Enter container
This command allows me to enter the container
docker exec -it matrix-synapse-1 /bin/bash
Create user
This commands allows me to create a user inside the container
register_new_matrix_user -c /data/homeserver.yaml http://localhost:8008