Rails server is still running in a new opened docker container

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.

I was stumped by the same problem for a bit until I figured out what was really going on. The real answer is further down... At first, you may want to try something like the following command in your docker-compose.yml file:

command: /bin/sh -c "rm -f /rails/tmp/pids/server.pid && rails server puma"

(I'm using Alpine and busybox to keep things tiny, so no bash! But the -c will work with bash too) This will delete the file if it exists so that you don't get stuck with the problem that the container keeps exiting and sitting there unable to run commands.

Unfortunately, that is NOT a good solution because you add an extra layer of /bin/sh ahead of the server and that prevents the server from getting the stop command. The result is that the docker stop commands won't give a graceful exit and the problem will always happen.

You could just run the rm command using docker-compose up to remove the file and then change the command back to the server and carry on.

However, the real answer is the create a simple docker-entry.sh file and make sure you call it using the exec form Entrypoint Documentation so that signals (like stop) get to the server process.

#!/bin/sh
set -e

if [ -f tmp/pids/server.pid ]; then
  rm tmp/pids/server.pid
fi

exec bundle exec "$@"

NOTE: we use exec on the last line to make sure that rails will be run as pid 1 (i.e. no extra shell) and thus get the signals to stop. And then in the Dockerfile (or the compose.yml) file add the entrypoint and command

# Get stuff running
ENTRYPOINT ["docker-entrypoint.sh"]
CMD ["rails", "server", "puma"]

And again, you MUST use the [] format so that it is exec'd instead of run as sh -c ""