pre-build libraries for Julia packages and create default git config

Build and install native dependencies of bundled Julia packages in base image. Reduces build time and space requirement for Julia packages (Dockerfile v13).

Also generating a default git config for users on first login. 

> Is there a PPA we can use?

I highly doubt it. It might be fairly easy to take the up-to-date Debian source from http://anonscm.debian.org/cgit/debian-science/packages/coinor-cbc.git/tree/ and put it in a PPA, but launchpad has been a PITA every time I've tried to use it.

> We just want to avoid compilation to the extent possible.

What for? Isn't there an automated build service that you can sign up for to build docker images for you? If not, rearrange the dockerfile so the time-consuming but rarely-changing parts are at the beginning and get cached.