Better handling of bind mounts in make-disk-image

The device of the bind mounts is incorrectly taken verbatim while making the image.
The root hierarchy is mounted at `config.rootMountDir` (`/mnt` by default) during the process,
and this commit prepends that path to the `device` in these cases.

Author: zaninime

lib/types/nodev.nix

@@ -46,15 +46,20 @@
     };
     _mount = diskoLib.mkMountOption {
       inherit config options;
-      default = lib.optionalAttrs (config.mountpoint != null) {
-        fs.${config.mountpoint} = ''
-          if ! findmnt ${config.fsType} "${rootMountPoint}${config.mountpoint}" > /dev/null 2>&1; then
-            mount -t ${config.fsType} "${config.device}" "${rootMountPoint}${config.mountpoint}" \
-            ${lib.concatMapStringsSep " " (opt: "-o ${opt}") config.mountOptions} \
-            -o X-mount.mkdir
-          fi
-        '';
-      };
+      default =
+        let
+          isBindMount = builtins.elem "bind" config.mountOptions;
+          device = if isBindMount then "${rootMountPoint}${config.device}" else config.device;
+        in
+        lib.optionalAttrs (config.mountpoint != null) {
+          fs.${config.mountpoint} = ''
+            if ! findmnt ${config.fsType} "${rootMountPoint}${config.mountpoint}" > /dev/null 2>&1; then
+              mount -t ${config.fsType} "${device}" "${rootMountPoint}${config.mountpoint}" \
+              ${lib.concatMapStringsSep " " (opt: "-o ${opt}") config.mountOptions} \
+              -o X-mount.mkdir
+            fi
+          '';
+        };
     };
     _unmount = diskoLib.mkUnmountOption {
       inherit config options;