Add a note about Services Environment Variables in the relationships Docs

[flovntp]

Priority

3 - Not urgent - can be addressed as team bandwidth permits

Where should the changes be made?

https://docs.upsun.com/create-apps/image-properties/relationships.html

What exactly should be updated?

Add a note about the Service environment variable as a follow up for someone defining a relationships:
I define my relationships, and then? how to interact with my service from the app container?

Optional: Additional context

No response