volumes: set hostPath based on volume id

this is required for the file browser to work which does operations
based on the id

fixes #789

src/paths.js

@@ -21,6 +21,8 @@ exports = module.exports = {
     PROVIDER_FILE: '/etc/cloudron/PROVIDER',
     SETUP_TOKEN_FILE: '/etc/cloudron/SETUP_TOKEN',
 
+    VOLUMES_MOUNT_DIR: '/mnt/volumes',
+
     PLATFORM_DATA_DIR: path.join(baseDir(), 'platformdata'),
     APPS_DATA_DIR: path.join(baseDir(), 'appsdata'),
     BOX_DATA_DIR: path.join(baseDir(), 'boxdata'),  // box data dir is part of box backup