* gnu/system/vm.scm (system-docker-image): Add #:shared-network? and
pass it to 'containerized-operating-system'.
(qemu-image):
* guix/scripts/system.scm (system-derivation-for-action): Pass
#:shared-network? to 'system-docker-image'.
* doc/guix.texi (Invoking guix system): Document it.