Set SINGULARITY_CACHEDIR to something on /volatile

[wdconinc]

Since SINGULARITY_CACHEDIR is in the pwd by default, it will fill up user quota pretty quickly. A safer approach is to store in /volatile/$USER/singularity or some such directory if on ifarm. This could happen transparently and revert back to pwd when /volatile/$USER/singularity unwriteable.

[wdconinc]

Proper dir is /volatile/halla/parity/$USER

[wdconinc]

Singularity always puts pull output into pwd, not in CACHEDIR. That complicates things a bit...