feat: adds Docker health check

[pedrolamas]

Adds a basic health check to the docker images.

[Copilot]

Pull request overview

Adds a lightweight Docker health check for Fluidd’s NGINX-based images by introducing a /healthz endpoint and wiring a Dockerfile HEALTHCHECK to poll it, with accompanying docs and CI tweaks.

Changes:

• Add location = /healthz in the NGINX config template returning 200 with plain-text ok.
• Add a Docker image HEALTHCHECK that polls http://127.0.0.1:${PORT}/healthz, and plumb PORT via build args/env.
• Update build workflow to pass PORT as a Docker build-arg (removing the prior sed-based template rewrite) and document the new health check.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
server/nginx/default.conf.template	Adds /healthz endpoint for local health probing.
Dockerfile	Introduces PORT env and Docker HEALTHCHECK polling /healthz.
.github/workflows/build.yml	Removes template rewrite step; passes PORT build-arg for image variants.
docs/docs/getting-started.md	Documents the health check and Compose service_healthy usage.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.