Add Support for init.d Autostart Configuration (for Docker Compatibility)

[chankahou0968]

Hello and thank you for maintaining ttyd — it’s an indispensable tool for lightweight web-based terminal access.

After installing ttyd via apt install ttyd, I noticed that only systemd-based autostart is provided. However, in Docker containers, systemd is typically unsupported or unavailable. This makes it difficult to configure ttyd to launch automatically within containerized environments.

Would you consider including init.d autostart support in future releases or documentation? Even an official init.d script would go a long way in enhancing ttyd's compatibility and ease of deployment in Docker and other minimalist setups.

Thanks again for your work and for considering this request!