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/SHARED_PROJECT_DEPLOYMENT.md

@@ -17,7 +17,7 @@ The AI Stack Deployer portal deploys **all user AI stacks to a single shared Dok
 │  📦 Portal Application: ai-stack-deployer-prod                 │
 │     ├─ Domain: portal.ai.flexinit.nl                           │
 │     ├─ Image: git.app.flexinit.nl/.../ai-stack-deployer:latest│
-│     └─ Env: SHARED_PROJECT_ID=${{project.SHARED_PROJECT_ID}}  │
+│     └─ Env: SHARED_PROJECT_ID=$${{project.SHARED_PROJECT_ID}}  │
 │                                                                 │
 │  ───────────────────────────────────────────────────────────── │
 │                                                                 │
@@ -56,9 +56,11 @@ When a user submits a stack name (e.g., "john-dev"), the portal:
 2. **These are set via Dokploy's project-level variables**:
    ```yaml
    environment:
-     - SHARED_PROJECT_ID=${{project.SHARED_PROJECT_ID}}
-     - SHARED_ENVIRONMENT_ID=${{project.SHARED_ENVIRONMENT_ID}}
+     - SHARED_PROJECT_ID=$${{project.SHARED_PROJECT_ID}}
+     - SHARED_ENVIRONMENT_ID=$${{project.SHARED_ENVIRONMENT_ID}}
    ```
+   
+   **Note**: The double `$$` is required to escape the dollar sign in Docker Compose.
 
 ### Step 2: Portal Deploys to Shared Project