cybersandbox: drop `USER 1000` from Dockerfile so compose doesn't need user: "0:0" override

[kdairatchi]

Problem

`cybersandbox/Dockerfile:150` ends with:

```dockerfile

── Drop to non-root (use UID to avoid passwd lookup issues) ─

USER 1000
```

This was intended as a hardening step, but it breaks compatibility with the upstream entrypoint contract.

Upstream `/opt/gem/run.sh` (which our overlay `entrypoint-cyber.sh` `exec`s) expects root at entry so it can:

1. `useradd` / `groupadd` the runtime user
2. `chown -R $USER:$USER /opt/jupyter`
3. Write `/etc/sudoers.d/$USER`

…and then it self-drops to `$USER` via `su -` before `exec`ing supervisord. Every supervised process (nginx, jupyter, code-server, mcp-hub, browser, VNC) ends up running as the non-root user anyway. The `USER 1000` directive in our Dockerfile is redundant hardening that actually breaks the container.

Current workaround

PR #16 works around this with a `user: "0:0"` override in `docker-compose.yaml`, plus a comment pointing at this issue.

Proper fix

1. Delete line 150 (`USER 1000`) and the preceding comment in `cybersandbox/Dockerfile`.
2. Rebuild the image, publish `:latest` via the existing `cybersandbox-build.yml` workflow (build + trivy + cosign + SBOM already wired up).
3. Drop `user: "0:0"` from `docker-compose.yaml` in a follow-up PR.
4. Verify: `docker compose up -d` → healthy → supervisord services list shows all processes still owned by `hunter`, not root.

Repro

On a clean WSL / Linux host pulling the current `:latest`:

```
docker pull ghcr.io/prowlrbot/cybersandbox:latest
docker run --rm ghcr.io/prowlrbot/cybersandbox:latest

... tool check ok ... "CyberSandbox Ready" ...

chown: changing ownership of '/opt/jupyter': Operation not permitted

container exits, docker restarts, loop forever

```

Acceptance

• `cybersandbox/Dockerfile` no longer contains `USER 1000`.
• New `:latest` published; `docker compose up -d` with a stripped compose (no `user:` override) reaches healthy.
• Inside the container, `ps -o user,comm` shows nginx/jupyter/code-server/mcp-hub all running as `hunter`, not root.
• PR fix+docs: make docker compose up -d actually work #16's `user: "0:0"` comment + override removed in the follow-up PR.

Related: #16