Switch to official images in readme

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Use official images on getting started guide

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Use official images in rails tutorial

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #392 from bfirsh/more-official-images

More official images
Don't use deprecated orchardup/python image

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #379 from docker/use-library-python-image-for-dockerfile

Use library python image for dockerfile
Use hostnames everywhere in docs, add YAML note and deprecate env.md

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>