You are using an onbuild image, so your working direcotry is mounted in the container image. This is very good for developing, since your app is updated in realtime when you edit your code, and your host system gets updated for example when you run a migration.
This also means that your host system tmp directory will be written with the pid file every time a server is running and will remain there if the server is not shut down correctly.
Just run this command from your host system:
sudo rm tmp/pids/server.pid
This can be a real pain when you are for example using foreman under docker-compose, since just pressing ctrl+c will not remove the pid file.