Use Docker for local development environment

- Update README and CLAUDE.md with Docker-based dev workflow
- Add Dockerfile for containerized deployment
- Add gunicorn to requirements.txt
- Dev uses volume mount for live code reloading

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -17,7 +17,7 @@ Kebuu is a spending tracker web app built with Flask. Currently in early develop
 - **Backend**: Flask 3.0, SQLAlchemy, Flask-Login
 - **Auth**: bcrypt password hashing, Altcha CAPTCHA (self-hosted)
 - **Forms**: Flask-WTF with CSRF protection
-- **Server**: Gunicorn (production), Flask dev server (local)
+- **Server**: Docker + Gunicorn (dev and prod)
 - **Database**: SQLite (dev), PostgreSQL (prod)
 
 ## Directory Structure
@@ -42,23 +42,24 @@ src/
 
 ## Commands
 
-### Local Development
-```bash
-cd src
-pip install -r requirements.txt
-python app.py
-```
-
-### Docker
+### Build
 ```bash
 docker build -t kebuu .
+```
+
+### Development
+```bash
+docker run -p 5000:5000 --env-file .env.dev -v $(pwd)/src:/app kebuu
+```
+
+### Production
+```bash
 docker run -p 5000:5000 --env-file .env.prod kebuu
 ```
 
 ### Testing
 ```bash
-cd src
-python -m unittest discover
+docker run --env-file .env.dev kebuu python -m unittest discover
 ```
 
 ## Environment Variables