Which specific Dockerfile instruction is used to define a command that Docker can run to check if the application inside a container is truly healthy and responsive?

The specific Dockerfile instruction used to define a command that Docker can run to check if the application inside a container is truly healthy and responsive is `HEALTHCHECK`. This instruction allows you to specify a command that Docker will execute periodically inside the running container to determine its operational status. If this specified command exits with a status code of `0`, it indicates a successful health check, meaning the application is healthy and responsive. An exit code of `1` signifies a failed health check, indicating the application is unhealthy or unresponsive. An exit code of `2` is reserved and should not be used. Docker uses this exit code to update the container's health status, which can then be monitored by users or orchestration systems. The `HEALTHCHECK` instruction accepts optional parameters to fin....