mount empty ~/.casa for casa6 cabs

[o-smirnov]

CASA6 (and older, possibly -- @bennahugo can you confirm?) looks for user configuration in ~/.casa.

This implies a potential unintended interaction with the user environment when running inside the container, compromising reproducibility.

The solution would be to mount an empty ~/.casa when invoking the container (and possibly provide a runtime option to override this, for users that really do want to mount their own). This possibly needs an extra stimela feature, which would fit in well under the cab management section.