Documentation for container usage

[jrlagrone]

SuperPOD

• downloading a container with enroot from a repository (e.g. dockerhub, ngc, etc)
• modifying an enroot container
• running jobs in an enroot container
• best practices for reproducibility

Both

• downloading a container with apptainer from a repository (e.g. dockerhub, ngc, etc)
• converting a local (not on a repository) docker container to apptainer (e.g. using docker2singularity)
• customizing an apptainer container
• running jobs in an apptainer container
• best practices for reproducibility