Auto-create and chown data dirs on first start

Entrypoint runs as root, ensures /data/{db,uploads,logo} and
/webdav-config exist with UID 1000 ownership, then drops privileges
via gosu. Removes the manual sudo chown step from the README and
makes a fresh docker compose up succeed without prep.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -3,7 +3,7 @@ FROM node:20-bookworm-slim
 WORKDIR /app
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    python3 make g++ \
+    python3 make g++ gosu \
   && rm -rf /var/lib/apt/lists/*
 
 COPY package.json ./
@@ -22,6 +22,10 @@ ENV NODE_ENV=production \
     DB_PATH=/data/db/app.db \
     WEBDAV_CONFIG_DIR=/webdav-config
 
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
 EXPOSE 3000
 
+ENTRYPOINT ["/entrypoint.sh"]
 CMD ["node", "src/server.js"]