Speedup image build process - fix timeouts

I was curious why the build process of our new containers takes so much time and therefore the ci job ultimately runs into a timeout.

Compared to the old plain PHP containers we used to build this is incredibly slow:

• See gitlab-ci-jobs for https://git.team23.de/docker/php/-/blob/master/7.4/Dockerfile for example: ~10-20min
• This repositories gitlab-ci-jobs: >=2h 😱

This MR makes small tweaks here and there in order to achieve similar performance when building the images.

• use more threads using -j "$(nproc)" (more info: https://stackoverflow.com/a/46957492)