composer.json for just core How do I get just Drupal 8 core using drupal-composer?

This would be very useful for core development and starting a new project without any assumption.
Sounds like you want drupal-console.  This tool can quickly create working Drupal 8 sites interactively or via command line. More info here.

Another quick alternative is to use DrupalVM and Varnish to spin up a fully working D8 site.  The DrupalVM defaults are to create a D8 sandbox site.

I believe that DrupalVM now has Docker support so you might be able to have a single VM and just spin up a new D8 box.