Dockerfiles can be a contentious topic in the Docker world. Their simplicity makes them extremely easy to grasp, but this simplicity can be a source of frustration for users familiar with the flexibility of typical build and configuration management tools. However, it has recently become possible add to significant flexibility to Dockerfiles with just a touch of trickery.

These tricks only work in the 1.8.X releases of Docker - the feature they rely on was sadly removed in later releases.