Support user Docker userns-remap

[olljanat]

It is possible to run containers on user namespace which improves security https://docs.docker.com/engine/security/userns-remap/

This PR includes all needed pre-requirements so users can enable that feature easily by just running:

sudo ros config set rancher.docker.userns_enabled true
sudo reboot

Functionality can be verified by starting some container as root user:

docker run -d --rm -u root --name nginx -p 80:80 nginx

and then checking processes. UID is 100000 instead of root when this feature is enabled.

$ ps -Af | grep nginx
100000    1956  1936  0 17:41 ?        00:00:00 nginx: master process nginx -g daemon off;
100101    2018  1956  0 17:41 ?        00:00:00 nginx: worker process

Another nice way is mount root file system inside of container and try write to it. It is read-only when this feature is enabled.

docker run -it -v /:/host bash bash
$ touch /host/test
touch: /host/test: Permission denied

This will need changed also to https://github.com/burmilla/burmilla.github.io/blob/master/content/docs/configuration/docker/_index.md

[tomaswarynyca]

Good contribution!