What do you mean? Do you really mean all of the settings? There are hundreds of them as you can see at /admin/settings. Do you mean the settings overridden with ENV variables? What you have should do that, I think.
But you can do something like this to get the settings set in the database:
docker exec -it app bash -c 'echo "SiteSetting.pluck(:name, :value)" |rails c'
To my surprise (though it made sense after thinking about it), this is just the ones that have been changed.