Allow services to be used across images from list of images

docker

#1

When you list images, for example:

docker:
  images:
    - ruby
    - memcache

You cna’t use memcache in your build because first build listed, the ruby one, hasn’t it installed. You need to have custom image with all services installed there. It would be super useful to have ability to just list images and use all services out of the box


#2