Docker: add .env.example, .dockerignore; polish README Docker section

- Add .env.example (referenced by README Quick Start; populates
  SECRET_KEY/POSTGRES_*/AUTH0_* used by docker-compose).
- Add .dockerignore so `COPY . .` no longer bakes .git, __pycache__,
  venv, local data, and secrets into the image.
- Rewrite README Docker Deployment into a single `docker compose
  up -d --build` flow on port 5021; fix local-dev access port 5000->5021.
- Drop obsolete `version: '3.8'` from docker-compose.yml.

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

README.md

@@ -58,26 +58,41 @@ A Reddit-style content aggregator that collects posts from multiple platforms (R
    ```
 
 7. **Access the application**
-   - Open browser to `http://localhost:5000`
+   - Open browser to `http://localhost:5021`
    - Create an account or browse anonymously
 
 ### Docker Deployment
 
-1. **Build the image**
+The included `docker-compose.yml` runs the app plus a PostgreSQL database, with persistent volumes for the database, collected posts, and backups.
+
+1. **Configure environment**
+   ```bash
+   cp .env.example .env
+   # Edit .env — at minimum set SECRET_KEY to a long random string
+   ```
+
+2. **Build and start the stack**
+   ```bash
+   docker compose up -d --build
+   ```
+
+3. **Access the application**
+   - Open browser to `http://localhost:5021`
+   - Create an account or browse anonymously
+
+Useful commands:
+   ```bash
+   docker compose logs -f app     # follow app logs
+   docker compose down             # stop the stack (keeps data volumes)
+   docker compose down -v          # stop and delete all data volumes
+   ```
+
+To publish the image to the registry instead of building locally:
    ```bash
    docker build -t git.scorpi.us/chelsea/balanceboard:latest .
-   ```
-
-2. **Push to registry**
-   ```bash
    docker push git.scorpi.us/chelsea/balanceboard:latest
    ```
 
-3. **Deploy with docker-compose**
-   ```bash
-   docker-compose up -d
-   ```
-
 See [DEPLOYMENT.md](DEPLOYMENT.md) for detailed deployment instructions.
 
 ## Configuration