[Build Todo] Linux Docker permissions

[jfallows]

Since Docker is a prerequisite for Build the Todo Application, some users will need to install Docker for the first time to go through the guide.

@ralyodio reported a permission problem requiring chmod 666 /var/run/docker.sock on Linux.

We may need to include a hint / warning to help readers to successfully complete the guide.

Note that it may also be sufficient to add the current user to the Linux docker group, and then re-login to gain the permissions.

[ralyodio]

Actually the proper way to fix is to add yourself to the docker group. Just be sure to use -a for add