Fix database not initializing in Docker (no db file created)

Root cause: DATABASE_URL used a relative path (file:./data/rackmapper.db).
Prisma CLI (migrate deploy) resolves relative SQLite paths from the
prisma/ schema directory -> /app/prisma/data/rackmapper.db, while the
Prisma Client at runtime resolves from CWD -> /app/data/rackmapper.db.
The migration ran against a different path than the bind mount, so no
database file ever appeared in /app/data (the mounted volume).

Fixes:
- Change DATABASE_URL to absolute path: file:/app/data/rackmapper.db
  everywhere (docker-compose, .env.example, UNRAID.md)
- Replace inline CMD with docker-entrypoint.sh:
    mkdir -p /app/data before migrating (safety net)
    npx prisma migrate deploy with set -e so failures are visible
    exec node dist/server/index.js
  This surfaces migration errors in docker logs instead of silently
  exiting, and ensures the data dir always exists before SQLite opens it
- Update .env.example to reflect plain ADMIN_PASSWORD and COOKIE_SECURE

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-entrypoint.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+set -e
+
+echo "[entrypoint] RackMapper starting..."
+
+# Ensure the data directory exists and is writable
+mkdir -p /app/data
+echo "[entrypoint] Data directory: $(ls -la /app/data)"
+
+# Run migrations (creates the SQLite file if it doesn't exist)
+echo "[entrypoint] Running database migrations..."
+npx prisma migrate deploy
+echo "[entrypoint] Migrations complete."
+
+# Start the server
+echo "[entrypoint] Starting server..."
+exec node dist/server/index.js