linux-container: Accept file systems with a UUID 'source'.

* gnu/system/linux-container.scm (containerized-operating-system)[user-file-systems]:
Check whether SOURCE is a string before calling 'string-prefix?'.

gnu/system/linux-container.scm

@@ -57,7 +57,8 @@ (define user-file-systems
                     (source (file-system-device fs)))
                 (or (string=? target (%store-prefix))
                     (string=? target "/")
-                    (string-prefix? "/dev/" source)
+                    (and (string? source)
+                         (string-prefix? "/dev/" source))
                     (string-prefix? "/dev" target)
                     (string-prefix? "/sys" target))))
             (operating-system-file-systems os)))