security: run Etherpad container as non-root user (fixes #7134) (#7287)

Change the Docker Compose user from "0:0" (root) to "5001:0" (etherpad user)
to follow security best practices and the principle of least privilege.

The Dockerfile already creates a non-root user 'etherpad' with UID 5001
and GID 0, so this change aligns the docker-compose.yml configuration
with the Dockerfile's security model.

Benefits:
- Reduces attack surface by not running as root
- Follows Docker security best practices
- Aligns with the Dockerfile's existing non-root user setup
- Compatible with OpenShift and other platforms that restrict root containers
- Maintains group permissions (GID 0) for volume access

Fixes #7134

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   app:
-    user: "0:0"
+    user: "5001:0"
     image: etherpad/etherpad:latest
     tty: true
     stdin_open: true