Add docker and gitlab ci support

This adds automatic compilation inside docker.
I currently have it as a public image on Docker Hub https://hub.docker.com/repository/docker/dmitriigalantsev/umr.
Give me your username there and I can add you to collaborators.

Potential improvements:

• Compile and upload docker image automatically when Dockerfile changes. See https://www.howtogeek.com/devops/how-to-build-docker-images-in-a-gitlab-ci-pipeline/
• Run tests as part of CI

Note: You will need to add an executor to UMR project on Gitlab.