Make the base image compatible with matplotlib

The MPLCONFIGDIR by default is '/', which is not writable by the user 'user'. By adding this line, it will work.

I tested it by manually installing matplotlib inside the container, 'docker commit' the change, and re-run.

I'm not sure whether the purpose of the base container 'ptone/jiffylab-base2' is to become more complete, but since matplotlib is a fairly basic thing to want to have, I added it here.