image: Add a shared-network? field.

* gnu/image.scm (<image>)[shared-network?]: New field.

gnu/image.scm

@@ -43,6 +43,7 @@
             image-compression?
             image-volatile-root?
             image-shared-store?
+            image-shared-network?
             image-substitutable?
 
             image-type
@@ -98,6 +99,8 @@
                       (default #t))
   (shared-store?      image-shared-store? ;boolean
                       (default #f))
+  (shared-network?    image-shared-network? ;boolean
+                      (default #f))
   (substitutable?     image-substitutable? ;boolean
                       (default #t)))