There are a couple of approaches you can use.
docker build --no-cacheto avoid using the cache of the Git clone.
git pull. So instead of running
python manage.py, you'd have something like
CMD cd /repo && git pull && python manage.pyor use a start script if things are more complex.
I tend to prefer 2. You can also run a cron job to update the code in your container, but that's a little more work and goes somewhat against the Docker philosophy.
I would recommend you checkout out the code on your host and
COPY it into the image. That way it will be updated whenever you make a change. Also, during development you can bind mount the source directory over the code directory in the container, meaning any changes are reflected immediately in the container.
A docker command for git repositories that checks for the last update would be very useful though!
Docker build command uses cache as long as a instruction string is exactly same as the one of cached image. So, if you write
RUN echo '2014122400' >/dev/null && git pull ...
On next update, you change as follows.
RUN echo '2014122501' >/dev/null && git pull ...
This can prevents docker from using cache.