nspawn: Don't mount read-only if we have a custom mount on root.

src/nspawn/nspawn-mount.c

@@ -989,6 +989,19 @@ int mount_custom(
         return 0;
 }
 
+bool has_custom_root_mount(const CustomMount *mounts, size_t n) {
+        size_t i;
+
+        for (i = 0; i < n; i++) {
+                const CustomMount *m = mounts + i;
+
+                if (path_equal(m->destination, "/"))
+                        return true;
+        }
+
+        return false;
+}
+
 static int setup_volatile_state(const char *directory, uid_t uid_shift, const char *selinux_apifs_context) {
 
         _cleanup_free_ char *buf = NULL;