I found the solution, here it is for the benefits of this great community.

When getting the image, una goes through the transcoder. In docker, this is done inside the 'php' container.

From within this container, it performs a call to a url that looks like http: //localhost/s/bx_persons_pictures/sc6rxpgsiibve3v9mzmd6tgefrskprun.jpg in order to fetch the image and resize it.

But from within the container, the hostname is lost (here it is localhost, it can be my-una-website or whatever). So the resize call fails, the resized picture is not resized nor stored in the DB. The avatar picture is not found, and not displayed.

In order to fix that, you need to specify the hostname to your the 'php'. This is done by a simple 'hosname' line in the docker-compose.yml:

1. version: '3'
2. services:
3. web:
4. hostname: localhost (or my-una-website or whatever)
5. image: nginx:latest
6. ports:
7. - "80:80"
8. ...