I am trying to setup promtail as part of my docker-compose setup to collect my Rails logs.

This is the promtail part of my docker-compose.yml:

  promtail:
    image: grafana/promtail:3.0.0
    volumes:
      - ./docker/promtail/config.yml:/etc/promtail/config.yml
      - ./log:/var/log/rails
    command: -config.file=/etc/promtail/config.yml

When I start the image:

docker-compose up promtail

I see the files on the image: /var/log/rails are updated:

docker-compose exec promtail tail /var/log/rails/development.log

Then I make some requests to my app, and new logs are added in my local files (host, outside the Docker):

tail ./logs/development.log

But the new logs are replicated in the same file in the Docker image:

docker-compose exec promtail tail /var/log/rails/development.log

I can continue generating logs through my Rails app, but the logs are not replicated in the log file in my image.

The interesting part is that if I execute this in the host (outside the Docker):

echo "XXX" >> ./log/development.log

Suddenly, all the logs are replicated in my Docker image.

How can I make sure the log files are replicated inside the Docker image when they are updated outside?