feat(docker): add a HEALTHCHECK to the image (#1394)

Report Docker application health using a bounded loopback HTTP probe.

Contribution by @14traits.

Dockerfile

@@ -39,6 +39,11 @@ COPY --chown=node:node . .
 ENV HOST=0.0.0.0
 ENV PORT=4173
 EXPOSE 4173
+# node:slim has no curl, so probe with Node's built-in fetch. 127.0.0.1 is
+# always on the Host allowlist, and the generous start period covers Vite's
+# first-run dependency optimisation.
+HEALTHCHECK --interval=30s --timeout=10s --start-period=30s --retries=3 \
+  CMD ["node", "-e", "fetch('http://127.0.0.1:'+(Number.parseInt(process.env.PORT,10)||4173)+'/').then((r)=>process.exit(r.ok?0:1),()=>process.exit(1))"]
 # The launcher is `npm run dev` plus one step: it trusts the container's
 # gateway address for Provider Settings, because that is where Docker delivers
 # the host's own browser from (never 127.0.0.1). See scripts/docker-start.mjs.

changelog.d/1394-docker-healthcheck.md

@@ -0,0 +1,2 @@
+- Report Docker container health with a local HTTP check using Node, with the
+  same port parsing as the launcher (John Rogers, @14traits, #1394).

docs/CURRENT-STATE.md

@@ -6228,3 +6228,12 @@ when their footprints touch or contain one another, never across a seam toleranc
 - `scripts/qa-form-fields.mjs` exercises native selections, number validity,
   calibration commit/cancel and stale-camera/disposal guards against an installed
   controller with fixture ports, plus default/Cyber and narrow translated layouts.
+
+## Docker health reporting
+
+The Docker image probes its local HTTP root with Node every 30 seconds, using
+the launcher's parsed `PORT` (default 4173). A successful HTTP response marks
+the container healthy. Checks time out after 10 seconds, with a 30-second
+startup grace period and three consecutive failures before reporting unhealthy.
+This reports app HTTP availability, not individual provider health, and does not
+configure automatic restarts.