Optimize CI workflow (3)

Thanks to all progress made in ut/pppp1718/quizfort@0be481a6, which was merged with master, the CI workflow now has static analysis for the front-end code and Docker throughout the whole workflow.

However, these changes also show the shortcomings of our CI workflow. The CI workflow now does a lot of things twice, such as compiling the code for the tests and then also for packaging again. Therefore, it makes sense to optimize the CI workflow even further. See !142 (merged) for more information.

Once this is merged, it will heavily improve performance.