change buildah storage to vfs due to a bug in buildah with overlayfs

Summary

There seems to be a bug in buildah with overlayfs in recent versions that shows up when building e.g. ubuntu:16.04 image, see, e.g., https://github.com/containers/buildah/issues/1837, https://github.com/containers/buildah/issues/2382 Proposed solution: use vfs instead of overlayfs with the drawback of slow builds and high storage space requiremenets

See also failing jobs: e.g. https://gitlab.dune-project.org/docker/ci/-/jobs/157565

closes #14 (closed)