Change docker link to products page

[markusjura]

This issue needs a change in conductr-doc and Typesafe.com

The instruction to install docker is referencing a link to http://docker.com. On http://docker.com it not obvious for a new user with what kind of product from Docker he should get started. Therefore we should link directly to the product page of the Docker tool: https://www.docker.com/products/docker

Here the user will find the download links for any OS.