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'.
* gnu/tests/docker.scm (run-docker-test): Add parameters. Load and run
docker container. Check response of guest guile.
(build-tarball&run-docker-test): New procedure.
(%test-docker): Use it.
[description]: Modify.
Ludovic Courtès