feat: enable Docker CLI inside container image

[krzemienski]

✨ Summary

Enable Docker CLI and Compose v2 inside the codex-universal container image for Ubuntu 24.04 by installing the docker.io client plus the docker-compose-v2 plugin in a dedicated ### DOCKER ### layer.
This guarantees docker compose … works out‑of‑the‑box when Codex tests repositories that rely on container orchestration.

Fixes #10

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🔍 Context

• Modern integration tests frequently orchestrate services (DBs, caches, side‑cars) with docker compose.
• The current image ships only language runtimes; any repo that invokes Docker fails immediately.
• On Ubuntu 24.04 LTS (“noble”) docker.io does not bundle Compose—the v2 CLI lives in docker-compose-v2. Installing both packages (no daemon, just CLI) unblocks container workflows while keeping the existing security model (CLI talks to a mounted socket; no privileged service starts inside the image).

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🛠️ What’s inside

File	Change
Dockerfile	Append apt-get install -y --no-install-recommends docker.io docker-compose-v2 in a new ### DOCKER ### layer

No other files or behaviours are modified.

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🚀 How to Test

# Build the updated image
docker build -t codex-universal-docker .

# Run it with access to the host Docker socket
docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock codex-universal-docker bash

# Inside the container:
docker --version          # → Docker version <X.Y.Z>, build …
docker compose version    # → Docker Compose version v2.<N>

Optional smoke‑test

echo -e 'FROM alpine\nCMD ["echo","hello"]' > Dockerfile
docker build -t hello-alpine .
docker run --rm hello-alpine   # → prints “hello”

All commands should succeed without “command not found” errors.

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📏 Impact & Size

• Adds ≈ 25 MB compressed to the final image.
• No privileged mode required; socket pass‑through remains sufficient.
• Zero breaking changes for projects that never call Docker.

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📝 Checklist

• docker.io + docker-compose-v2 installed in Dockerfile
• docker compose verified inside rebuilt image
• PR references Issue Install Docker CLI in codex-universal image #10 (“Fixes Install Docker CLI in codex-universal image #10”)

[seuros]

Excuse me, but running docker inside docker it not possible as far as know in sandbox mode.