Kroki and PlantUML with local install does not render image

Hopefully someone can point me in the right direction.
I’m trying to install Kroki (or PlantUML) locallly on another server than Gitlab itself.
PlantUML and Kroki are installed with docker.

The weird thing is that Gitlab does not display the rendered image just the “image missing” icon, but the url is correct. If I click the link, I get the diagram in a new tab.
But it is not displayed embedded.

So, I have this view:

But if I click the broken image icon, I get a new tab with the correct diagram:

If I change the Kroki config to point at the image is displayed embedded.
The same happens for pure PlantUML integrations.

Any suggestions on how to get this displayed inline?