Follow GitLab CI best practices

Follow GitLab CI best practices

This commit changes few things, implementing some of the GitLab CI known best practices:

1. Explicitly pointing the docker:git image to a specific version.

   Leaving the floating one we're risking that a buggy release of new Docker image will be randomly introduced to our pipelines. It's also best to make the Docker Client (used by the job from within the docker:git image) to be at the same version as the Docker Engine (running in the docker:dind service container).

2. Replacing ci-job-token with CI_REGISTRY_USER and CI_JOB_TOKEN with CI_REGISTRY_PASSWORD makes the script more bulletproff. Yes, at this moment these variables contain the same values. But they are dedicated for such usage and if GitLab CI will ever decide to switch how Registry access should be done, the dedicated variables will folow the proper way.