'make halt-mocknet' command

Consistent with this,
I would find it convenient to be able to halt a mocknet
(for resuming later without needing a re-bootstrap)
without needing a copy-and-paste.

The term 'halt' currently seems reasonable to me as a short term which doesn't replace the different behaviour of the existing make stop-mocknet;
as always, feedback is welcome.

(A related question (thoughts welcome) is whether a make command would be appropriate for
docker compose -f build/docker/docker-compose.yml exec thornode sh
which I also find useful, and if so what it should be called given that make cli-mocknet exists for a different command.)