Add warning when installing custom container

[kkauder]

Is your feature request related to a problem? Please describe.
On linux, singularity pull creates a multi-GB cache in ~/.singularity

Describe the solution you'd like
That's easy to avoid, just set SINGULARITY_CACHEDIR. The user should know this.
I would just add "This can add a large cache file in your $HOME, to avoid export (or setenv) SINGULARITY_CACHEDIR to the desired one" to the -n, -c, and -v options. I'd be happy to do that, but it's an. unwieldy sentence, it maybe best to group these three and have the sentence just once before them ("The following options...").
Let me know which way, I'll make the PR

Describe alternatives you've considered
I mean, one could add another option and set the variable inside the script, but that seems like overkill.

Additional context
Add any other context or screenshots about the feature request here.

[wdconinc]

I'm quite happy to add some warnings to those running singularity or docker without cvmfs (and provide a link to how to install cvmfs). Those are the 'annoying' modes to support. If we provide better redirect to running singularity against cvmfs I also won't feel as bad about implementing some functionality only for singularity against cvfms (e.g. #2).

So, something like:

Notice: singularity stores large files in \$SINGULARITY_CACHEDIR, which is set to:
  $SINGULARITY_CACHEDIR
You are responsible for periodically clearing the contents of this directory.

and

Notice: You are NOT using cvmfs, and will need to upgrade eic-shell manually.
To use cvmfs, check the Getting Started section at https://cvmfs.readthedocs.io.

[kkauder]

I agree to the above. One possible addition: The docker installation has a similar problem, not as pronounced because it doesn't happen in farm nodes with small home directories. But users need to regularly eic-shell --upgrade AND clean up with docker image prune