Improve CI efficiency

[forgottosave]

Description

The CI introduced in #19 can be drastically improved in efficiency, by creating a custom docker/container image based on fedora:latest where all dependencies required in most builds are already installed.

Tasks

• Create custom container including all dependencies
• Rebuild image automatically every once in a while so it stays up to date
• As mentioned by @COM8, maybe it can be hosted inside the GitHub container registry, this needs further investigation (https://github.blog/news-insights/product-news/introducing-github-container-registry/)
• Another side thing that would also improve "efficiency": Maybe properly define, what CI should run on which event (currently a lot of jobs run twice on a push to a PR)