Could not check for JavaScript support. Please check manually if your webserver serves `.mjs` files using the JavaScript MIME type

[koelle25]

@henningsieh

services:
  app:
    image: nextcloud:28.0.3
    container_name: nextcloud28-app
    restart: unless-stopped
    hostname: cloud.sieh.org

Try taking out the hostname: cloud.sieh.org line. Due to it the check runs internally in the docker container against the included apache webserver instead of against your external nginx proxy where you added the mjs mimetype.

Also not that this is merely a WARNING, not an error and as such should only be of a bigger concern if something with the Nextcloud is actually not working.