Docker: Why You See Unhealthy Without Specifying a Health Check

There are two possible scenarios when you see Up (unhealthy) without explicitly defining HEALTHCHECK:

Inherited Health Checks from Base Image

Some official or custom base images may already define a HEALTHCHECK directive in their Dockerfile. If your Docker image is built on such a base image, the health check will propagate to your final container unless explicitly overridden or disabled.

For example:

dockerfile

FROM mysql:latest  # The MySQL base image includes a default HEALTHCHECK

To inspect if a base image has a health check:

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docker inspect <base-image-id> | grep -A 5 '"Healthcheck"'

Orchestrator or Monitoring Tool Integration

If you're running Docker containers with orchestration tools like Docker Swarm, Kubernetes, or using Docker with third-party monitoring systems, the orchestrator or tool might simulate health checks (e.g., by probing network endpoints). This can indirectly cause Up (unhealthy) statuses.

How Docker Handles Default Status Without Health Checks

Without Health Check:

If no health check is specified, Docker only tracks the basic running state of the container.

Status examples:

Up — The container process is running.

Exited — The container has stopped (with an exit code).

With Health Check (Inherited or Defined):

If a health check is defined or inherited:

Docker periodically runs the health check command.

The container status becomes:

healthy — If the health check succeeds.

unhealthy — If the health check fails multiple times consecutively.

How to Remove or Disable Inherited Health Checks

If the HEALTHCHECK directive is coming from a base image, you can disable it by adding the following in your Dockerfile:

dockerfile

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HEALTHCHECK NONE

This will completely remove any inherited health check from the base image.

Next Steps for Debugging

Check for Inherited Health Checks: Inspect the image you’re using to see if it has a health check:

docker inspect <image-name>

Override or Remove Health Checks: Use the HEALTHCHECK NONE directive in your Dockerfile or configure a custom health check to suit your app.

Log Analysis: Examine container logs (docker logs <container-id>) to understand why the runtime might appear "unhealthy."