Add ccache

[Anders Logg (Chalmers)]

Add ccache to speed up recompilation. Steps:

1. Install Ubuntu package ccache.
2. Add to .bash_profile: export PATH=/usr/lib/ccache:${PATH}

[Prof Garth Wells]

I'm not sure we want the images to be the 'kitchen sink'. Users can easily install packages with apt-get.

[Anders Logg (Chalmers)]

If it's not bloat and if it's not harmful I don't see the problem.

[Prof Garth Wells]

It is bloat if everyone adds their favourite packages. Wait until someone asks for emacs . . . it's big.

[Anders Logg (Chalmers)]

It shouldn't be everyone's favorite package. We should have a sensible discussion on which packages/tools are useful for all/many developers (to share knowledge / best practices).

[Prof Garth Wells]

Dockerfile best practice guidelines:

https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#/avoid-installing-unnecessary-packages

[Jack Hale]

For 250kB extra this seems like a valuable addition. As a point of comparison, I've rejected Jan's request to install xterm. Fixed in pull request #17.