get CVMFS cache into a Docker volume

[tpdownes]

A great/easy solution for users would make use of Docker volumes for the CVMFS cache. To be able to share volumes among containers (and thereby document a single command to invoke containers from images) we should explore the use of an alien cache. Going by memory, the command would then look something like

docker run -it --privileged -v cvmfs:/var/cache/cvmfs -v /Users:/Users ligo/shell

where cvmfs is the reusable volume stored on the user's workstation.