Hello!

The argument REGISTER-CLOSURE? of the SYSTEM-DOCKER-IMAGE procedure can be
removed and its value computed automatically, since the operating-system
definition is available in its context.  When the operating-system definition
does not contain the GUIX-SERVICE-TYPE, do not register the closure in the
database of Guix, as it takes time and doesn't serve a purpose.

The time saving is close to 2 minutes on my machine for every test using
a very minimal OS configuration and building it with `guix system
docker-image my-config.scm'.

Thank you,

Maxim