add continuous integration support

• Shared GitLab runner with Docker executor was set up
  • Runner watches all projects in group zavolan_group that contain a GitLab CI/CD config file (.gitlab-ci.yml) in their root directory; according to the configuration, changes in the repository trigger a Docker container to be started up and user-defined commands to be executed inside the container; status and logs of these runs can be accessed via the "CI/CD -> Pipelines"; see GitLab CI/CD tutorial for more details
  • In order to execute singularity and docker commands inside the Docker container, runner was configured to run Docker in privileged mode
  • CI/CD runs are executed on a dedicated virtual machine with 4 CPUs, 8 Gb of RAM and ~90 Gb of disk space
• CI/CD config file with a basic test (snakemake -n) was added; more tests can be added later by modifying/amending the config file
• A badge showing the CI/CD pipeline's latest status was added to the project's landing page; it was added via the "General" settings (under "Badges") with info obtained from the "CI/CD" settings (under "General Pipelines" > "Pipeline status")

Closes #27 (closed)