Docker rails

Hey everyone - I'm submitting this PR to get some early feedback. I'm wondering if the post is clear and has a flow to it. It's quite hard to explain the setup since there is a ton of moving parts just for a basic infrastracture. Also, a lot of the setup is repeated per container so I did my best to skip things that were already mentioned in the post.

Who knew devops could be so complicated? :dancer: 
Where does this docker image come from?