feat: docker container and kubernetes integration

Packaging the application as a docker image.

Closes #98 (closed), #205 (closed), #232

TODO

• Volumes for public und upload folder
• Firebase Config (either as secret file or with env variables)
• Migrate to Kubernetes configuration
• Fix: To see deployment progress for your environments, make sure you are deploying to $KUBE_NAMESPACE and annotating with app.gitlab.com/app=$CI_PROJECT_PATH_SLUG and app.gitlab.com/env=$CI_ENVIRONMENT_SLUG
• Handle secrets
• Pull correct images
• Fix pipeline and build a docker image
• Integrate GitLab's Kubernetes and App Review integration
• Logging, monitoring, security and backups

TODO in separate MRs

• Add documentation (#226 (closed))
• Complete nginx configuration (header, caching, security, etc.) (#227 (closed))
• Update node (#228 (closed))
• Remove babel from server build (#228 (closed))
• Rebuild images once a week (#229 (closed))