vm: 'system-docker-image' provides an entry point.

This simplifies use of images created with 'guix system docker-image'.

* gnu/system/vm.scm (system-docker-image)[boot-program]: New variable.
[os]: Add it to the GC roots.
[build]: Pass #:entry-point to 'build-docker-image'.
* gnu/tests/docker.scm (run-docker-system-test): New procedure.
(%test-docker-system): New variable.
* doc/guix.texi (Invoking guix system): Remove GUIX_NEW_SYSTEM hack and
'--entrypoint' from the example.  Mention 'docker create', 'docker
start', and 'docker exec'.