fix: escape dollar signs in Dokploy project-level variables

Docker Compose interprets $ as variable substitution, so we need to escape
Dokploy's project-level variable syntax by doubling the dollar sign.

Changes:
- docker-compose.*.yml: ${{project.VAR}} → $${{project.VAR}}
- Updated DOKPLOY_DEPLOYMENT.md with correct syntax and explanation
- Updated SHARED_PROJECT_DEPLOYMENT.md with correct syntax and explanation

This fixes the 'You may need to escape any $ with another $' error when
deploying via Dokploy.

Evidence: Tested in Dokploy deployment - error resolved with $$ escaping.

docs/DOKPLOY_DEPLOYMENT.md

@@ -143,13 +143,15 @@ In the shared project (`ai-stack-portal`), add these **project-level environment
 
 The portal's docker-compose files use Dokploy's variable syntax to reference these:
 
-```yaml
-environment:
-  - SHARED_PROJECT_ID=${{project.SHARED_PROJECT_ID}}
-  - SHARED_ENVIRONMENT_ID=${{project.SHARED_ENVIRONMENT_ID}}
-```
+   ```yaml
+   environment:
+     - SHARED_PROJECT_ID=$${{project.SHARED_PROJECT_ID}}
+     - SHARED_ENVIRONMENT_ID=$${{project.SHARED_ENVIRONMENT_ID}}
+   ```
 
-**This syntax `${{project.VARIABLE}}` tells Dokploy**: "Get this value from the project-level environment variables"
+**This syntax `$${{project.VARIABLE}}` tells Dokploy**: "Get this value from the project-level environment variables"
+   
+**Note**: The double `$$` is required to escape the dollar sign in Docker Compose files.
 
 ### Important Notes