fix: resolve permission error when saving gallery config in Docker

The container was running as 'nextjs' (UID 1001) but bind-mounted
./content from the host has host-user ownership, making it unwritable.

Solution: start as root, fix /app/content ownership in entrypoint,
then drop privileges to nextjs via su-exec before running the app.

docker-entrypoint.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+# Ensure the content directory is writable by the nextjs user (UID 1001).
+# This is needed when ./content is bind-mounted from the host.
+
+# Fix ownership of content directory so the app can save config
+if [ -d /app/content ]; then
+  chown -R nextjs:nodejs /app/content 2>/dev/null || true
+fi
+
+# Drop privileges and run the app as nextjs
+exec su-exec nextjs node server.js