Use preconfigured docker image

Rationale: every time we are starting build we are installing the same dependencies which take time. When dedicated image with already installed dependencies will be user we can speed up builds (there is speedup from 7 into 5 minutes).

Image is build using Dockerfile from this repository: https://gitlab.com/eurvalve/gitlab-ci-docker-images. I will improve it to make docker build and push into docker hub using gitlab-ci.