Speedup CI by not updating debs index

We expect all dependencies to be baked into the image, we do install the yarn/composer dependencies though.

The image can be found at: https://github.com/DOMjudge/domjudge-packaging/tree/main/docker-gitlabci

Author: vmcj

.github/jobs/baseinstall.sh

@@ -30,7 +30,6 @@ section_end
 
 section_start "Install JS frontend dependencies"
 cd webapp
-apt-get update; apt-get install -y yarnpkg
 yarnpkg install
 cd ..
 section_end