Move Docker guide from workshop specific page to a general tutorial

[ferrouswheel]

[vsbogd]

I would also suggest get Helsinki's workshop docker image and simplify it to use use as local network environment. Then we will have two docker images:

• to play with SingNet locally (modified workshop docker)
• to play with SingNet in Kovan or Ropsten (modified tutorials docker) Then we could be made tutorials network independent, i.e. each tutorial could be used in both environments.