add chown warning

docs/install.md

@@ -269,6 +269,8 @@ services:
     restart: unless-stopped
 ```
 
+If using a non-root user when running the docker version, be sure to chown the server.yml, user.db, and cache.db files to the same uid/gid.
+
 Alternatively, you may wish to build a customized Docker image that can be run with fewer command-line arguments and without delivering the configuration file separately.
 ```
 FROM binwiederhier/ntfy