When executing the command "docker volume list", I noticed that there were a few other volumes present. I then did a "docker-compose down" and then a "docker volume prune" which got rid of unused volumes. Then I restarted all containers with "docker-compose up -d" , and now there was only one volume left. After a restore with PGADMIN, the volume was populated.

So, somehow docker was mixing up volumes. Getting rid of all volumes solved the problem.