Dockerfiles which define images for CircleCI

The images are pre-built in OBS
<https://build.opensuse.org/project/show/devel:openQA:ci>

base - used to determine list of actual dependencies with versions
installed by openQA-devel package, then used as a base to install
cached packages inside CI.
The script tools/ci/build_dependencies.sh will update ci-packages.txt file
with list of new packages inside base image after installing openQA-devel.
The script .circeci/build_cache.sh is used inside circleci (both remote and
local) to create testing environment by installing packages from
tools/ci/ci-packages.txt onto base image.

These files normally don't need to be maintained, unless processes described
above must be tweaked now and then.
