linux-container: ‘containerized-operating-system’ provides ‘loopback’ service.

Fixes #420.

Previously, in systems produced by ‘guix system container’, the
‘loopback’ Shepherd service would fail to start with EEXIST, because the
“lo” interface already exists in the container.  This commit provides a
fake ‘loopback’ Shepherd service that does nothing.

* gnu/system/linux-container.scm (dummy-networking-service-type): Change
to honor its configuration value.
(containerized-operating-system)[services-to-drop]: Add
‘static-networking-service-type’ in the non-‘shared-network?’ case.
[services-to-add]: Add ‘dummy-networking-service-type’ for loopback in
the non-‘shared-network?’ case.

Reported-by: Antonio Carlos Padoan Junior <acpadoanjr@yahoo.com.br>
Change-Id: I326b5f3b5e08f67b6613c186497c44b1e887b02e

gnu/system/linux-container.scm

@@ -78,12 +78,13 @@ from OS that are needed on the bare metal and not in a container."
 (define dummy-networking-service-type
   (shepherd-service-type
    'dummy-networking
-   (const (shepherd-service
+   (lambda (provision)
+     (shepherd-service
       (documentation "Provide loopback and networking without actually
 doing anything.")
-           (provision '(loopback networking))
+      (provision provision)
       (start #~(const #t))))
-   #f
+   '(loopback networking)
    (description "Provide loopback and networking without actually doing
 anything.  This service is used by guest systems running in containers, where
 networking support is provided by the host.")))
@@ -135,7 +136,7 @@ containerized OS.  EXTRA-FILE-SYSTEMS is a list of file systems to add to OS."
                  dhcp-client-service-type
                  network-manager-service-type
                  connman-service-type)
-                (list))))
+                (list static-networking-service-type)))) ;loopback
 
   (define services-to-add
     ;; Many Guix services depend on a 'networking' shepherd
@@ -143,8 +144,10 @@ containerized OS.  EXTRA-FILE-SYSTEMS is a list of file systems to add to OS."
     ;; service when we are sure that networking is already set up
     ;; in the host and can be used.  That prevents double setup.
     (if shared-network?
-        (list (service dummy-networking-service-type))
-        '()))
+        (list (service dummy-networking-service-type
+                       '(loopback networking)))
+        (list (service dummy-networking-service-type
+                       '(loopback)))))
 
   (define os-with-base-essential-services
     (operating-system