Make it extra clear that the `docker` group is root-equivalent.

The following section on TCP access to docker has a warning that docker is root-equivalent.  Unfortunately, people who were interested in local but not TCP access (like me) won't read that section and therefore won't read that warning that applies to the docker group too.  If you think it's better to copy that warning to the "sudo and the docker Group" section, that'd be fine with me too. (Each section should have at least a sentence of warning in it, IMHO.)