linux-container: Do not add %CONTAINER-FILE-SYSTEMS to Docker image OSes.

linux-container: Do not add %CONTAINER-FILE-SYSTEMS to Docker image OSes.

Previously, 'guix system docker-image' would end up providing an OS that
would try to mount all of %CONTAINER-FILE-SYSTEMS as well as /gnu/store,
which is bound to fail in unprivileged Docker.

This patch makes it so that 'guix system container' still gets those
file systems, but 'guix system docker-image' doesn't.

* gnu/system/linux-container.scm (containerized-operating-system): Add
 #:extra-file-systems parameter and honor it.  Do not include
 %STORE-MAPPING and SHARED-NETWORK-FILE-MAPPINGS.
(container-script): Add %STORE-MAPPING and optionally NETWORK-MAPPINGS
to MAPPINGS and pass #:extra-file-systems.