pin Dockerfile to use jessie-based python
pin Dockerfile to use jessie-based python

 * The python:3.5-slim docker tag was recently repointed to a debian
   stretch-based image.  Until MFR has been verified to work on
   stretch, pin to the jessie-based image it has been using.

felliott/modular-file-renderer,CenterForOpenScience/modular-file-renderer,CenterForOpenScience/modular-file-renderer,felliott/modular-file-renderer,felliott/modular-file-renderer,felliott/modular-file-renderer,CenterForOpenScience/modular-file-renderer,CenterForOpenScience/modular-file-renderer