Building off a base image for dev/prod images

gistfile1.txt

GOAL: Use docker to build some images:
 - a base image
 - a dev image, from base, with extra dev deps, and the source code mounted as a host volume
 - a prod image, from base, with src explicitly ADDed into the image as a stand alone executable
SUBGOALS:
 - make use of docker's cache for faster building
 
A base image:

$ cat base.docker
# A context-less dockerfile that builds a base image
FROM ubuntu:precise
RUN echo "deb http://archive.me.com/ubuntu/ precise main" > /etc/apt/sources.list.d/mine.list                                                                                            
RUN apt-get update                                                                                                                                                                                                
RUN apt-get install -y --no-install-recommends some stuff
RUN apt-get install -y --no-install-recommends more stuff
RUN mkdir -p /srv/app/{src,log,etc,scripts}
$
$ docker build -t base - < base.docker
...
That works fine, no problems

For the dev image, it should install dev dependancies, which include python packages from a requirements.txt file in the src tree. But the source tree will be a volume at run time, so won't be available at build time.

So, AIUI I can do 3 things

1) Write a straight up Dockerfile (which can have context), which ADDs the requirements.txt into the image, and pip installed from that build. Problem: you can only have 1 Dockerfile (build context is only supported in that case), and I would want to use that for the prod image as well...

2) Script the changes to base image and commit the final image as the dev image. Problems: no nice docker caching of changes, full rebuild every time.

3) Rather than use requirements.txt, embed the actual dependancies in a context-less dockerfile. Problem: very different process for developers, prefer to stick with python standard of requirements.txt