Limited Time Offer! For Less Than the Cost of a Starbucks Coffee, Access All DevOpsSchool Videos on YouTube Unlimitedly. Master DevOps, SRE, DevSecOps Skills! Enroll Now Source – ostechnix.com Watchtower is a free, open source application that allows you to monitor the running Docker containers and updates them automatically if it finds any changes in their base images. When watchtower finds that a running container needs to be updated, it will gracefully stop the running container by sending it a SIGTERM signal.