What is the purpose of the `HEALTHCHECK` instruction in a Dockerfile?

The HEALTHCHECK instruction in a Dockerfile is specifically designed to define a command that monitors the health of the container at runtime. This instruction provides a way to specify how Docker should determine if a container is healthy or not while it is running. By implementing a health check, you can set up a command that Docker will execute at regular intervals, and based on the success or failure of this command, Docker can mark the container as healthy, unhealthy, or starting.

When a container is marked as unhealthy, orchestrators like Docker Swarm or Kubernetes can take action, such as restarting the container or notifying you of the problem. This functionality is crucial for maintaining the reliability of applications deployed in containers, as it allows for automatic recovery and ensures that services remain available to users.

The other options do not relate to the primary purpose of the HEALTHCHECK instruction: running commands during the image build phase, specifying the base image, or configuring environment variables are different functionalities within the Dockerfile that serve other purposes.