feat(cli): Mount user config directory in container

[braun-viathan]

What?

This PR enhances run_in_container.sh by optionally mounting the user's local .codex configuration directory into the Docker container. This allows configuration and session persistence between container restarts.

Fixes: #789

Key Changes:

• Adds an optional --config flag.
• Default configuration path: $HOME/.codex if --config not provided.
• Updates usage examples and documentation.

Why?

Currently, users lose Codex CLI configurations (approval modes, presets, logs) between container runs, as no persistent storage is provided. This change aligns Linux Docker behavior with other environments and improves the overall developer experience.

How?

• Implemented optional bind mount based on the existence of the provided config directory.
• Ensured backward compatibility: no breaking changes for existing users who do not provide a config path.

Manual Testing Steps:

1. Build the container:

   ./codex-cli/scripts/build_container.sh

2. Execute Codex CLI with config mounted:

   ./codex-cli/scripts/run_in_container.sh \
     --work-dir "$(pwd)" \
     --config ~/.codex \
     "codex --version"

3. Confirm that the mounted config is active (e.g., correct approval mode loaded).

Tested locally with success.

Checklist:

• Implementation
• Manual testing done
• Examples updated
• Awaiting feedback if further documentation is needed in README.md.