this is more of a Docker than a Deis question, but a Docker image is composed of layers. Each tagged image keeps an ancestry list of all of the layers it requires. In your case, if you run 5 docker containers on the same host on top of ubuntu:14.04, it will initially pull the parent's layers (ubuntu:14.04), then it will pull its own layers. They all share the same layers, which is why it's so powerful. You get the exact same execution environment every time.