Dokploy domains were configured to route to internal port 80, but the
Node standalone server listened on 3000, causing Bad Gateway. Align
the container's PORT/EXPOSE with Dokploy's routing instead.
The standalone build runs on macOS and bundles darwin-arm64 native
sharp binaries. These crash immediately on the Alpine Linux container,
causing the Node process to exit and Dokploy's reverse proxy to return
a 502 Bad Gateway.
This adds a RUN step to remove the macOS binaries and install the
correct @img/sharp-linuxmusl-x64 package at build time.
