I know this post is a few years old but i recently also ran into the unable to access Kannel admin commands, after some debugging, some coffee and a few bangs of my head against the wall i realised that i had installed Kannel into my host and not the container, the localhost cannot be contacted unless you add the --network=host flag to the docker run command that is provided. So use this:
docker run --network=host -d -p 80:80 playsms/playsms
The way i worked this out was by connected to the docker container, running curl (which needs to be installed) and then running
i soon realised that my container couldn’t see my local install so didn’t know where to look, the --network=host flag fixed that instantly.
I hope this helps anyone else out in the future