In case someone encounter the same problem, here is the workaround I ended up with.
I built a custom seafile-mc docker image with the following Dockerfile:
FROM seafileltd/seafile-mc:latest
RUN apt update && apt upgrade -y && apt install -y python python-configparser
I then changed the docker-compose.yml to use the resulting image instead of the official one .
Following the instructions at seafile-manual/docker/6.3%20upgrade%20to%207.0.md then worked as intended.
If somebody has a better solution, I’m all ears !
Regards,
David