fix: install linux-musl sharp binaries in Docker (fixes 502 on Alpine)

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.

Dockerfile

@@ -12,6 +12,15 @@ ENV HOSTNAME=0.0.0.0
 # Copy the standalone build artifact (server.js, .next/, public/)
 COPY build /app
 
+# The standalone build was created on macOS (darwin-arm64), so it bundles
+# macOS-only native binaries for sharp (image optimisation). These crash
+# on Linux/Alpine. Replace them with the correct musl-linux-x64 binaries.
+RUN rm -rf node_modules/@img/sharp-darwin-arm64 \
+           node_modules/@img/sharp-libvips-darwin-arm64 && \
+    npm install --no-save --platform=linuxmusl --arch=x64 \
+      @img/sharp-linuxmusl-x64@0.34.5 && \
+    rm -rf /tmp/* /root/.npm
+
 EXPOSE 3000
 
 CMD ["node", "server.js"]