I had a problem when running the seed. I couldn't telnet to it from an external box. I tried to play with the firewall but that wasn't it, the culprit was in the run.sh script:

PORTS="2001"
if [[ -f .env ]]; then
source .env
fi

What's happening is, after PORT is set to 2001, it gets overridden by the value from the .env file. In this case, it's empty (as instructed in this post).

PORTS=
DOCKER_NAME=witness

So when running the docker container, the resulting command that starts the docker is:
docker run -v /dev/shm:/shm -v /home/user/steem-docker/data:/steem -d --name witness -t steem

The fix is to set PORTS=2001 in the .env file. And we end up with the proper command being executed, with an open port allowing public access to the seed node:
docker run -p0.0.0.0:2001:2001 -v /dev/shm:/shm -v /home/user/steem-docker/data:/steem -d --name witness -t steem