Troubleshoot: Double check docker installation

Let's add a troubleshooting section about when the user runs the building tool (`sbt`) and don't have permissions to the docker's system socket.

The user may be tempted to use `sudo` but that leads to other problems. Like #217 in #216.
Add some commands so that the user can quickly check his permission. Like the `docker ps` must not return permission errors.
Point the user to docker's documentation if that's the case. Like https://docs.docker.com/engine/install/linux-postinstall/

The issue names no documentation file, so start by locating the troubleshooting or build documentation around the sbt entry point. Check the Docker Linux post-install guidance and verify the proposed permission check with docker ps; done means the section explains the permission error, avoids recommending sudo, and links to the Docker documentation.

Written by the indexing model from the issue text.