Maintenance and adds config for external reverse proxy.

- 📚Update docs for proxy_host
- 🔨Add proxy_host/app_url variable fixes #12
- ⬆Update base image to 7.0.5

Questions? Join our Discord server! https://discord.me/hassioaddons

[Full Changelog][changelog]

[changelog]: https://github.com/hassio-addons/addon-bookstack/compare/v0.2.2...v0.2.3