feat: gate the healthcheck listen file to docker only

Dockerfile

@@ -57,6 +57,11 @@ COPY --from=go-base /bin/hp_healthcheck /bin/hp_healthcheck
 HEALTHCHECK --interval=30s --timeout=5s --start-period=5s --retries=3 \
 	CMD ["/bin/hp_healthcheck"]
 
+# Tells Headplane to publish its loopback healthcheck URL to this
+# file on startup; `hp_healthcheck` reads it. Docker-only — native
+# installs don't ship a consumer.
+ENV HEADPLANE_LISTEN_FILE=/tmp/headplane-listen
+
 WORKDIR /app
 CMD [ "/app/build/server/index.js" ]
 
@@ -73,5 +78,7 @@ COPY --from=go-base /bin/hp_healthcheck /bin/hp_healthcheck
 HEALTHCHECK --interval=30s --timeout=5s --start-period=5s --retries=3 \
 	CMD ["/bin/hp_healthcheck"]
 
+ENV HEADPLANE_LISTEN_FILE=/tmp/headplane-listen
+
 WORKDIR /app
 CMD [ "node", "/app/build/server/index.js" ]

app/server/main.ts

@@ -42,17 +42,26 @@ if (certPath || keyPath) {
   }
 }
 
-// Read by the bundled Docker healthcheck. Includes the basename
+// `HEADPLANE_LISTEN_FILE` is a Docker-specific contract: the
-// (`__PREFIX__`) so the Go binary can stay completely dumb — just
+// Dockerfile sets it to `/tmp/headplane-listen` so the bundled
-// GET whatever URL is in this file. `/tmp` is writable in
+// `hp_healthcheck` binary can discover the URL to probe. Native
-// distroless and in every dev shell we care about.
+// installs don't ship a consumer, so we just skip writing anything
-const healthURL = `${tls ? "https" : "http"}://127.0.0.1:${config.server.port}${__PREFIX__}/healthz`;
+// if the var isn't set.
+const listenFilePath = process.env.HEADPLANE_LISTEN_FILE;
+const listenFile = listenFilePath
+  ? {
+      path: listenFilePath,
+      // Full URL including `__PREFIX__` so the Go binary can GET it
+      // verbatim — no path joining, no basename knowledge needed.
+      url: `${tls ? "https" : "http"}://127.0.0.1:${config.server.port}${__PREFIX__}/healthz`,
+    }
+  : undefined;
 
 startHttpServer({
   host: config.server.host,
   port: config.server.port,
   tls,
-  listenFile: { path: "/tmp/headplane-listen", url: healthURL },
+  listenFile,
   listener: composeListener({
     basename: __PREFIX__,
     staticRoot: clientDir,