* gnu/system/vm.scm (system-docker-image): New procedure.
* guix/scripts/system.scm (system-derivation-for-action): Add a case for
docker-image, and in that case, call system-docker-image.
(show-help): Document docker-image.
(guix-system): Parse arguments for docker-image.
* doc/guix.texi (Invoking guix system): Document "guix system
docker-image".
* gnu/system/examples/docker-image.tmpl: New file.
