I use a command like this to keep cncjs running on my Raspberry Pi 3:

docker run -p 80:8000 --restart=always -d --privileged --name cncjs rohan/rpi-cncjs

Here is the full Dockerfile:

FROM hypriot/rpi-node:6.5

RUN npm install -g cncjs --unsafe-perm

EXPOSE 8000
CMD ["cnc", "-vv", "--allow-remote-access"]