docs: secret key generation

README.md

@@ -269,10 +269,18 @@ The project includes these Docker files:
    ```
 
 2. **Generate a secure secret key:**
+   
+   **With Elixir/Phoenix installed:**
    ```bash
-   # Run this locally to generate a new secret
    mix phx.gen.secret
    ```
+   
+   **Without Elixir/Phoenix (Linux/Unix):**
+   ```bash
+   dd if=/dev/random bs=1 count=64 status=none | base64 -w0 | cut -c1-64
+   ```
+   
+   > **Note**: The SECRET_KEY_BASE must be a cryptographically random string that's at least 64 characters long. Phoenix uses it to sign session cookies, CSRF tokens, and other security-sensitive data.
 
 3. **Database Configuration**: The default setup includes:
    - PostgreSQL 15 container