Shorten the build-time for docker builds

[whotwagner]

Currently the complete docker image is build at once. This takes a lot of time for each build. We could shorten the build time by inheriting from a pre-built image.

[ernstleierzopf]

As I have been working on the docker configurations on the weekend, I have seen that updating the system and installing some new packages took quite a long time. However, I am not sure how to solve this as pretty much any time we create an prebuilt Image, the packages get outdated.

Also the fact that all of these builds are parallelized increases the build time. I assume the Jenkins Machine is not having enough cores to handle 12-14 concurrent processes, where about half of them are heavy build tasks.

Still the "Available Configs" test takes much longer.

I also noticed that using caches does not work with the builds if you update anything like for example the anisble-project. These changes do not get caught and are simply skipped when using caches.

[ernstleierzopf]

However, I have another suggestion which might reduce the total build time drastically.
I feel comfortable to only run the docker build tests in development and main. Package-wise we do not have so many breaking changes with every pull request. Errors in the development branch could be fixed via hotfixes.

In addition to that, I could parallelize runConfAvailableTest by accepting an argument with the config which has to be tested.
This would reduce the build time for pull requests by about 8 minutes I think!