Add Docker support with Gunicorn, SQLite volume persistence, and optional PostgreSQL

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -52,3 +52,27 @@ default company settings/account mapping. No demo business data is created.
 The frontend logs in via the FunZone admin endpoint
 (`/api/auth/token/admin/`) and reuses the returned access token for this service.
 Make sure `SECRET_KEY` here is identical to the FunZone backend's.
+
+## Docker
+
+SQLite (default, zero extra services):
+
+```bash
+docker compose up -d --build
+```
+
+The API listens on **http://localhost:8010** (`/health`, `/api/accounting/...`).
+
+Set `SECRET_KEY` to match the FunZone backend before starting:
+
+```bash
+SECRET_KEY=your-shared-secret docker compose up -d --build
+```
+
+Optional PostgreSQL instead of SQLite:
+
+```bash
+docker compose --profile postgres up -d --build
+```
+
+Then set `DATABASE_URL=postgresql://accounting_user:accounting_password@accounting-db:5432/accounting_db` in your environment or `.env` file.