Support e.g. `SINGULARITY_OPTIONS="--writable --fakeroot" eic-shell`

Use case: enable write access to the container with writable --fakeroot. This will also require that the user has write access to the directory in which the container is stored, so not on cvmfs.

assigned to @wdconinc

enabled an automatic merge when the pipeline for 1ac88c46 succeeds

merged