Adjust build jobs and build Docker image as part of CI
A Docker image based on the official Nginx image is automatically built for the master branch and uploaded to GitLab's registry:
https://gitlab.com/arsnova/arsnova-webclient/container_registry
All build jobs have been adjusted, so they are no longer dependent on specific runners.
Refs arsnova/arsnova#124.
Edited by Daniel Gerhardt