Can't yet set PACKTIVITY_CONTAINER_RUNTIME to apptainer as that requires a packtivity update.
* Add APPTAINER_CACHEDIR as lxplus9 uses apptainer.
* Set SINGULARITY_CACHEDIR to be APPTAINER_CACHEDIR.
* Add notice to loging to private cotnainer registiries before use.
- Avoid issues with login problems later.
Partially addresses Issue #124
Can't yet set
PACKTIVITY_CONTAINER_RUNTIMEtoapptaineras that requires apacktivityupdate.