CI: Use custom docker image
The previous CI pipeline always built the required tools (repoman, git, overlint, pkgcheck) and its dependencies on each CI run. This is very inefficient and makes the pipeline take very long even for small changes.
This commit replaces it with the newly built custom docker image, providing a synced portage tree as well as the required tools already installed. This reduces the time required to run the CI pipeline drastically.
Edited by Adrian Schollmeyer