Brain dump after lesson

[clelange]

• clean up docker on laptop with CVMFS instructions
• check which docker-with-cvmfs approaches work on a Linux laptop
• add sketch for docker-in-docker procedure
• clean up Singularity section (once it works again)
• add Singularity on HTCondor example
• explain when to use which kind of base image
• add ML image example as further reading material