Troubleshoot: Double check docker installation

[FabioPinheiro]

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/