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 fine-tune its behavior. The general syntax is `HEALTHCHECK [OPTIONS] CMD command`.

Here are the important options:

`--interval=DURATION`: This parameter sets how often Docker should run the health check command. `DURATION` is a time value, for example, `30s` for 30 seconds or `5m` for 5 minutes. By default, it's 30 seconds.

`--timeout=DURATION`: This parameter defines the maximum amount of time `DURATION` (e.g., `3s` for 3 seconds) that the health check command is allowed to run before it is considered to have failed. If the command exceeds this duration, the check is marked as failed. The default timeout is 30 seconds.

`--start-period=DURATION`: This parameter provides an initial grace period, `DURATION`, for containers to start up and initialize before health check failures are counted towards the unhealthy status. During this period, if a health check fails, it won't contribute to the `retries` count. However, if a health check succeeds during this period, the container is considered started and healthy. The default is 0 seconds.

`--retries=N`: This parameter specifies the number `N` of consecutive failed health checks after which the container is considered `unhealthy`. `N` is a positive integer. For example, if `retries=3`, the container will be marked unhealthy only after three successive failures. The default is 3.

For example, to check if a web server running on `localhost` is responding to HTTP requests every 5 minutes, with a 3-second timeout, the instruction would look like this: `HEALTHCHECK --interval=5m --timeout=3s CMD curl -f http://localhost/ || exit 1`. Here, `curl -f http://localhost/` attempts to fetch the URL, and if it fails (e.g., `curl` returns a non-zero exit code), the `|| exit 1` ensures the health check command itself exits with `1`, signaling a failure to Docker. Docker then monitors this status and can report if the container, and thus the application within it, is truly healthy and responsive.