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.

dependency_bot provides an image with a container runtime environment, git and
few other tools. It is used in tools/ci/config.yml to
tools/ci/build_dependencies.sh

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