Add container requirements (psycopg3 + gunicorn)

requirements-docker.txt

@@ -0,0 +1,39 @@
+# Container dependency set for LEGO Instructions Manager.
+# Mirrors requirements.txt minus the Windows/dev-only bits (waitress,
+# pytest, flask-debugtoolbar), plus the Postgres driver and gunicorn.
+
+# Core Framework
+Flask>=3.0.0,<4.0.0
+Werkzeug>=3.0.0,<4.0.0
+
+# Database
+Flask-SQLAlchemy>=3.1.1,<4.0.0
+Flask-Migrate>=4.0.5,<5.0.0
+SQLAlchemy>=2.0.35,<3.0.0
+psycopg[binary]>=3.2.0,<4.0.0   # Postgres driver (postgresql+psycopg://)
+
+# Authentication
+Flask-Login>=0.6.3,<1.0.0
+Flask-Bcrypt>=1.0.1,<2.0.0
+
+# Forms & Validation
+Flask-WTF>=1.2.1,<2.0.0
+WTForms>=3.1.1,<4.0.0
+email-validator>=2.1.1,<3.0.0
+
+# HTTP Requests (for Brickset API)
+requests>=2.31.0,<3.0.0
+
+# File Handling
+Pillow>=11.0.0
+PyPDF2>=3.0.1,<4.0.0
+PyMuPDF>=1.24.0
+
+# Environment Variables
+python-dotenv>=1.0.0,<2.0.0
+
+# Date/Time Utilities
+python-dateutil>=2.8.2,<3.0.0
+
+# Production Server (Linux)
+gunicorn>=22.0.0,<24.0.0