[dist] Reduce build context of docker builds

Every docker image build has a build context that contains files needed
to build the image and a docker file that contains the recipe for the
image build.

By reducing the build context we decrease the overhead of a docker build
quite a bit and thus speed things up (see table below).

In addition clearer seperation of the various docker files we have and
their dependencies, eg. contrib/start_old_tests is only needed by
Dockerfile.old-test-suite.

docker image    before   | after
-------------------------|----------
base:           175.8 MB |  4.096 kB
backend:        175.8 MB |  7.025 MB
frontend-base:  175.8 MB | 18.940 MB
mariadb:        175.8 MB |  2.560 kB
memcached:      175.8 MB |  2.048 kB
old-test-suite: 175.8 MB | 18.940 MB
                         |
build time*:   0m16.364s | 0m1.459s

* running 'time rake docker:maintainer:rebuild' with no source changes
  applied (using cached docker images)