fix(docker): make /app/data writable by node user on bind-mounted volumes

The container ran as the unprivileged "node" user, but bind-mounted host
directories (e.g. ./data:/app/data in docker-compose or an Unraid appdata
folder) are mounted with root ownership. The node user therefore could not
create trxtd.db in /app/data, failing at startup with:

  Error: unable to open database file
      at file:///app/server/db.mjs:12

Fix:
- Add docker-entrypoint.sh that mkdir/chown /app/data to node:node on
  container start, then drops privileges back to node via su-exec
- Install su-exec in the runtime stage (apk add --no-cache su-exec)
- Remove USER node so the entrypoint runs as root and can fix ownership,
  with privilege dropping handled inside the entrypoint instead
- Wire the script up as ENTRYPOINT, keeping CMD as the app itself

The mount path itself is unchanged (/app/data, host ./data). Verified with
a real bind mount: /health responds ok and trxtd.db/-shm/-wal are created
in the mounted directory.

Dockerfile

@@ -13,27 +13,30 @@ FROM node:22-alpine
 WORKDIR /app
 ENV NODE_ENV=production
 
-# production dependencies only (ws)
+# production dependencies only (ws) + su-exec for privilege dropping
 COPY package.json package-lock.json ./
-RUN npm ci --omit=dev && npm cache clean --force
+RUN npm ci --omit=dev && npm cache clean --force && \
+    apk add --no-cache su-exec
 
 COPY server/server.mjs server/server.mjs
 COPY server/db.mjs server/db.mjs
 COPY shared/ shared/
 COPY --from=build /app/dist dist/
+COPY docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
+RUN chmod +x /usr/local/bin/docker-entrypoint.sh
 
 # persistent account/progress database (mount a volume here on the host)
-RUN mkdir -p /app/data && chown -R node:node /app/data
+RUN mkdir -p /app/data
 VOLUME ["/app/data"]
 
-# run as unprivileged user
-USER node
-
 EXPOSE 3001
 ENV PORT=3001
 ENV DATA_DIR=/app/data
 
+# The entrypoint fixes permissions on bind-mounted /app/data and drops
+# privileges to the unprivileged "node" user before running the app.
+ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
+CMD ["node", "server/server.mjs"]
+
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
   CMD node -e "fetch('http://localhost:3001/health').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
-
-CMD ["node", "server/server.mjs"]

docker-entrypoint.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+
+# Ensure the data directory exists and is owned by the node user.
+# This fixes permission issues with bind-mounted volumes (e.g. ./data:/app/data)
+# where the host directory is owned by root but the container runs as node.
+mkdir -p /app/data
+chown -R node:node /app/data 2>/dev/null || true
+
+# Drop privileges and run the actual application as node
+exec su-exec node "$@"