Add warning when installing custom container

**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.