vm: 'system-docker-image' calls 'sync' before rebooting.

Previously we could end up silently building truncated tarballs.

* gnu/system/vm.scm (system-docker-image)[build]: Add call to 'sync'.

gnu/system/vm.scm

@@ -526,7 +526,10 @@ should set REGISTER-CLOSURES? to #f."
                  #$os-drv
                  #:compressor '(#+(file-append gzip "/bin/gzip") "-9n")
                  #:creation-time (make-time time-utc 0 1)
-                 #:transformations `((,root-directory -> ""))))))))
+                 #:transformations `((,root-directory -> "")))
+
+                ;; Make sure the tarball is fully written before rebooting.
+                (sync))))))
     (expression->derivation-in-linux-vm
      name build
      #:make-disk-image? #f