Draft: contrib: (re)generate toolbx container for hacking on nmci

Add script to generate or refresh toolbx container for hacking on NM CI.

TL;DR:

$ git switch dj/toolbx
$ contrib/toolbox/rebuild-nmci-toolbox
(...)
$ toolbox enter nmci-fedora
$ pytest nmci/test-nmci.py

See the new file contrib/nmci-toolbox/README.md (link after MR is merged) for usage and notes.

@RunTests:pass