add CI stub for building multiarch images

[rokroskar]

With SwissDataScienceCenter/renkulab-docker#212 the renku base images will be built also for the arm64 architecture. For users with apple silicon, this will provide a huge boost for running renku sessions locally. However, for this we should also provide a way to build the project images via gitlab-ci. Since the builds add quite a bit of time to the CI job, we should probably just include a commented-out stub that would allow whoever is interested to enable building multiarch images.