Ummmmm, why do you think that is hanging there? Maybe is just the last message in the start-up.

I did a quick test on a fresh linux box following the instructions on the URL you posted:

After running the command in step 1 ("docker run -it --name iif_developer_edition ......") I get:

....
....
(expression)  Chunk 1 is now extendable. 

1 row(s) retrieved.



Database closed.

>>>    Starting up the IBM Informix Database (dev) ... 
*** Startup of dev SUCCESS ***

>>>    Type exit to quit the Startup Shell
>>>       This will stop the container

>>>    For interactive shell run:
>>>      docker exec -it 71280b101fc2 bash

>>>    To start the container run:
>>>      docker start 71280b101fc2 

>>>    To safely shutdown the container run:
>>>      docker stop 71280b101fc2 

IDS-12.10 dev: starting mqtt listener on port 27883
starting rest listener on port 27018
starting mongo listener on port 27017

And it waits there, but it is running, everything is properly initialized.

If you attach to the docker container (from a second shell, run "docker ps -a -q" to find the container id, and then "docker attach %container_id%) you can check the engine status.

root@irk21:/tmp# docker ps -a -q
71280b101fc2
770421a85b4c
root@irk21:/tmp# docker attach  71280b101fc2

....
....
IDS-12.10 dev: pwd    
/home/informix
IDS-12.10 dev: ls
data  ifx_dev.env
IDS-12.10 dev:    
IDS-12.10 dev: onstat -

IBM Informix Dynamic Server Version 12.10.FC8DE -- On-Line -- Up 00:09:46 -- 148084 Kbytes

IDS-12.10 dev: dbaccess sysmaster -

Database selected.

> ^C
IDS-12.10 dev:          

It should be up an running and all listeners ready to accept connections.

As answered already, you need to attach to the running docker container after you start and exit the shell. In the description page (https://hub.docker.com/r/ibmcom/informix-developer-database/), you can see this step:
4 - To attach to the Informix Docker container (shell)
docker exec -it iif_developer_edition bash