add initial varnish Dockerfile
add apt-get clean to Dockerfiles

I'm finding it reduces the image size:

mysql: 365.3 MB to 321.6 MB
postgis: 625.2 MB to 523 MB
varnish: 473.7 MB to 418 MB

It *has* to be in the same RUN as the install or it will actually make
the virtual image size go up (it make sense, it saves state after every
command).
Merge branch 'varnish'