Configure Docker for nested cloud-agent builds

[amdfaa]

Fixes N/A

Summary:

• Install Docker, a Docker buildx package, fuse-overlayfs, and the current Python venv support package during Ubuntu cloud-agent bootstrap.
• Configure /etc/docker/daemon.json to enable BuildKit and use the fuse-overlayfs storage driver.
• Restart or launch Docker in both systemd and non-systemd agents, then verify the active storage driver and buildx availability.

Startup failure diagnosis:

• A subagent inspected /tmp/cursor/async-install/install-user.status, /tmp/cursor/async-install/install-user.log, and /tmp/cursor/async-install/install-user.sh.
• The failing startup command was [ -d .venv ] || python3 -m venv .venv from the generated environment install script.
• The log showed ensurepip is not available and recommended apt install python3.12-venv; Docker/fuse-overlayfs setup did not run in that failed startup path.
• The bootstrap now installs the matching pythonX.Y-venv package before Python setup so Ubuntu cloud agents have venv/ensurepip support available.

Validation:

• bash -n codex_setup.sh
• git diff --check -- codex_setup.sh
• lintrunner -a codex_setup.sh could not run because the existing environment setup failed before installing lintrunner (python3 -m venv .venv failed due missing python3.12-venv).

Authored with Claude.