docs: add warnings about $ character in Docker Compose passwords

- Add clear warnings in .env.example about $ variable substitution
- Update DEPLOYMENT_GUIDE.md with password generation commands that exclude $
- Add troubleshooting section for Docker Compose variable substitution errors
- Provide solutions: avoid $, escape as $$, or use quotes

Fixes issue where passwords containing $ cause Docker Compose warnings
and potential authentication failures.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

DEPLOYMENT_GUIDE.md

@@ -58,13 +58,18 @@ Generate secure values:
 # JWT Secret
 openssl rand -base64 64
 
-# Database Password
-openssl rand -base64 32
+# Database Password (avoid $ character - see warning below)
+openssl rand -base64 32 | tr -d '$'
 
-# Redis Password
-openssl rand -base64 32
+# Redis Password (avoid $ character - see warning below)
+openssl rand -base64 32 | tr -d '$'
 ```
 
+⚠️ **PASSWORD WARNING**: Docker Compose interprets `$` as variable substitution. Either:
+- Avoid `$` in passwords (recommended - use the commands above)
+- Escape `$` as `$$` (e.g., `Pass$$word` instead of `Pass$word`)
+- Quote the entire value: `DB_PASSWORD='Pass$word'` (less reliable)
+
 Update `.env` with:
 - `JWT_SECRET` - Authentication secret (REQUIRED - generate a secure random value)
 - `DB_PASSWORD` - PostgreSQL password
@@ -432,6 +437,17 @@ FRONTEND_PORT=3002
 BACKEND_PORT=3003
 ```
 
+#### Docker Compose Variable Substitution Errors
+If you see warnings like:
+```
+WARN[0000] The "fgbf" variable is not set. Defaulting to a blank string.
+```
+
+This means your password contains `$` which Docker Compose interprets as a variable. Solutions:
+1. **Best**: Generate passwords without `$`: `openssl rand -base64 32 | tr -d '$'`
+2. **Alternative**: Escape `$` as `$$` in your .env file
+3. **Example**: `DB_PASSWORD=Pass@#$$fgbf` instead of `DB_PASSWORD=Pass@#$fgbf`
+
 #### Permission Errors
 ```bash
 # Fix ownership