cvmfs-xyz: share cache between containers

[sbinet]

currently, 2 cvmfs-atlas containers won't share their cache even if they run the same exact command atlas read-gen-event.py.

one way to fix this is to put a squid container in front of the cvmfs-xyz containers.

another way is to VOLUME-mount CVMFS_CACHE_BASE=/var/scratch/cvmfs in all cvmfs-xyz images and provide a data-only container exporting that VOLUME.

AFAICT, all the possible solutions will require a 2-container (or pod-like) setup.

[lukasheinrich]

does this work perhaps? seems like at least for NFS shares specifying the VOLUME with the ':shared' postfix works

moby/moby#4213

can't test right now since hepsw/cvmfs-atlas:latest is hangint while trying to mount :-/