Add "Run the Docker daemon as a non-root user (Rootless mode)":
`engine/security/rootless.md`
The content is based on https://github.com/moby/moby/blob/master/docs/rootless.md
`rootless.md` in `moby/moby` will be replaced of the link to
the `docs.docker.com` page compiled from `rootless.md` in this repo.
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
Akihiro Suda