Support Docker-native healthchecks in the scratch image #49

Problem

The official Docker image uses FROM scratch and contains no shell, wget, or curl. Although ZenNotes exposes /healthz, this prevents conventional Docker Compose healthchecks from working.

For self-hosted deployments using Coolify or other Compose-based platforms, there's currently no straightforward way to expose native Docker health status without customizing the image.

Proposal

Could the server binary provide a built-in healthcheck command?

For example:

healthcheck:
  test: ["CMD", "/zennotes-server", "healthcheck"]
  interval: 30s
  timeout: 5s
  retries: 3
  start_period: 30s

The command could check the existing /healthz endpoint and return a nonzero exit code when unhealthy.

This would preserve the minimal scratch image without introducing additional executables or dependencies.

An optional Dockerfile HEALTHCHECK instruction or documented Compose example would also be helpful.

Open to alternative approaches that provide Docker-native health status while retaining the minimal image.