Add Docker containerization

[achendev]

Summary

• ignore /docker/root files
• mount root directory in compose
• document root bind mount
• copy entire /etc, /home, and /root in init_creds.sh
• write env-supplied credentials to /var/cld/creds/creds_env with CLD_CFG_ prefix
• default to static creds file if no env vars supplied
• document env credential usage in README
• ensure a non-containerized creds file is converted to creds_static
• clean up cron setup invocation
• remove deprecated Compose version field

Testing

• docker-compose config (failed: command not found)
• docker compose config (failed: command not found)

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https://chatgpt.com/codex/tasks/task_e_6843680480848330be817b2d43a7b635