Docker auto-sets HOSTNAME to the container id; Next standalone's server.js
binds process.env.HOSTNAME, so it listened on the container hostname and the
compose healthcheck (TCP 127.0.0.1:3000) was refused → frontend container
chronically "(unhealthy)" (cosmetic: Caddy reaches it by service name and the
site serves 200). Pin HOSTNAME=0.0.0.0 — the canonical Next.js Docker fix.
Diagnosed on prod 2026-06-05.