Fix Docker build and add labels test data

- Fix Docker build issues:
  * Add missing system dependencies (build-essential, python3-dev, libpq-dev)
  * Fix requirements.txt formatting (separate Flask-Session and openpyxl)
  * Update openpyxl version to 3.1.5 (3.10.0 was invalid)
- Set proper folder permissions for Docker write access (app/ and data/)
- Add comprehensive test data for labels module:
  * 8 sample orders (TEST-ORD-001 through TEST-ORD-008)
  * Mix of printed/unprinted orders for testing
  * Various product types, customers, and delivery dates
  * Ready for QZ Tray printing functionality testing
- Include test data generation scripts for future use
- Application now fully containerized and ready for labels/printing testing

requirements.txt

@@ -9,4 +9,5 @@ Markdown==3.5.1
 APScheduler==3.10.4
 reportlab==4.0.7
 python-barcode==0.15.1
-Flask-Session==0.8.0openpyxl==3.10.0
+Flask-Session==0.8.0
+openpyxl==3.1.5