Use non-conflicting UID/GID for nonroot (e.g. 9999) — GID 999 is often taken by systemd-journal

### Summary

Examples that create an app user with **UID/GID 999** can fail once the image installs packages that pull in **systemd**, because Debian/Ubuntu postinst scripts commonly create the **`systemd-journal`** group with **GID 999**. After that, `groupadd --gid 999 …` exits with **code 4** (“GID already exists”).

### Suggestion

In Docker examples that hard-code a non-root user, prefer a **high, uncommon** UID/GID pair (e.g. **9999**) or omit fixed IDs and let `useradd`/`groupadd` allocate, and document why 999 is risky in “full” images.

### Motivation

`systemd-journal` on GID 999 is **very common** on Debian-derived images once systemd is present, so **999** is a poor default for custom application users in recipes that may combine `uv`/Python slim with desktop/browser or other systemd-using stacks.

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Assisted by Cursor

Inspect the Docker examples that create a non-root user with UID/GID 999, then check how package installation can reserve GID 999 for systemd-journal. Done means the affected examples use a non-conflicting or allocated ID and explain why 999 is risky.

Written by the indexing model from the issue text.