text: Title: Sort dependencies in requirements.txt/environment.yml to increase cache hit
  Body: ### Proposed change
  <!-- Use this section to describe the feature you'd like to be added. -->
  Sort the contents of `environment.yml` and /or `requirements.txt` before copying it to the image. The goal here is to make two repos that both require `numpy` and `pandas` but write them in different order in one of the two files mentioned share a cache layer.
  
  Comes via https://github.com/jupyter/repo2docker/pull/743#discussion_r304020569 and follow up comment about first doing a survey.
  
  ### Alternative options
  
  Do nothing.
  
  
  ### Who would use this feature?
  <!-- Describe the audience for this feature. This information will affect who chooses to work on the feature with you. -->
  People who build repositories that use a (very) common set of dependencies.
  
  ### How much effort will adding it take?
  <!-- Try to estimate how much work adding this feature will require. This information will affect who chooses to work on the feature with you. -->
  The change itself would be easy to implement. The main effort is in determining if this would benefit anyone or if it is very unlikely. In which case we should not implement this.
  
  ### Who can do this work?
  <!-- What skills are needed? Who can be recruited to add this feature? This information will affect who chooses to work on the feature with you. -->
  People with scripting skills who can put together a script to look at https://archive.analytics.mybinder.org/ to find example repositories to analyse and then compare the `environment.yml`/`requirements.txt` in these repositories.
  