Currently, the Varnish cache is not purged when editing related content because of missing relation-<content-id> tag. Relevant documentation: https://github.com/ezsystems/ezplatform-http-cache/blob/97689d1f5182bc80aea55dc9fcd771ddd8cc53df/docs/using_tags.md#tags-in-use-in-this-bundle.

Steps to reproduce:

1. Set up eZ Platform installation with Varnish using xkey VCL.
2. In Media/Images, create a new Image Content Object named "Test Image".
3. Under "Home", create a new Article Content Object named "Test Article". In the "Image" Field for this Content Object, select "".
4. Access your_site.example/Test-Article. It should correctly display the Article. There should be a link leading to "your_site.example/Media/Images/Test-Image" with the text "Test Image".
5. Now, rename "Test Image" to "Test Image Edited" using Platform UI.
6. Access your_site.example/Test-Article again. There will be a link leading to "your_site.example/Media/Images/Test-Image" with the text "Test Image". Instead, there should be a link leading to "your_site.example/Media/Images/Test-Image-Edited" with the text "Test Image Edited".