Documenting how to deploy a Docker image (first pass)

I've added a section to the 'using flynn' section about how to deploy a docker container. I've renamed the current section that was called release to redeploy, because I felt it was a more accurate title, and doesn't confuse with the 'release' cli argument.

Comments welcome. -- I feel looking up the image id using these two curl requests is super clunky, but I wanted to know if it's possible. -- and it is. First downloading the whole thing to be able to do 'docker images --no-trunc' to see the id is also kinda' silly.. 

Ultimately I think this lookup of the id is something that can probably be added in code, but I have not really looked at that yet.