Support ALLOWED_FRAME_URLS as a Runtime Environment Variable

[ceviixx]

Describe the feature or enhancement

Currently, ALLOWED_FRAME_URLS can only be configured at build time, which makes it difficult to use iframes without additional proxy adjustments. According to the documentation (https://umami.is/docs/environment-variables#allowed_frame_urls), this variable must be set during the build process.

Would it be possible to support ALLOWED_FRAME_URLS as a runtime environment variable instead?

This would allow configuring it directly in a docker-compose.yml file without having to build a custom image, greatly simplifying the setup and deployment process.

Thank you! 🙌