Add CI pipeline for Plymouth
We can use GitLab's CI/CD support to run a build.
Currently we have two CI jobs in the main pipeline:
- a Debian stable (Buster) build
- a Fedora (30) build
Both jobs are not the fastest possible, as they need to download and install all the dependencies. Making this faster would imply creating our own CI images with Docker and uploading them into the registry. Still, it's not exceedingly slow.