Ensure Docker usage is possible while using eic-shell (it's not currently)

In the tutorial, Docker and Singularity are mentioned on same footing, i.e. a reasonable person would assume that either would work with the image(s) provided. However, if Singularity is not present on the system, the installation will fail. Proposal: make the choice of Docker vs Singularity explicit at install time.

Reasoning -- making Singularity a hard requirement limits the scope of usage of eic-shell. Many users are heavily invested into Docker on their workstations and laptops and won't want to install yet another container system, with all sorts of related bloat.