feat: add self reference variables

apps/docs/content/docs/core/variables.mdx

@@ -1,21 +1,29 @@
 ---
 title: Environment Variables
-description: "Dokploy allows you to create and manage shared environment variables for your projects."
+description: "Dokploy allows you to create and manage shared and service-level environment variables for your projects."
 ---
 
 import { Callout } from "fumadocs-ui/components/callout";
 
-
 ## Overview
 
-Shared environment variables allow you to:
+Environment variables in Dokploy allow you to:
-- Define values once and use them across multiple services
+- Define configuration once and reuse it
-- Maintain configuration consistency between services
+- Share values across multiple services
+- Reference values from within the same service
 - Centrally manage sensitive information
 
-<ImageZoom src="/assets/images/shared-variables.png" width={800} height={630} alt='Shared variables panel' className="rounded-lg" />
+<ImageZoom src="/assets/images/shared-variables.png" width={800} height={630} alt='Environment variables panel' className="rounded-lg" />
 
-## Practical Example
+## Defining Variables
+
+You can declare environment variables either:
+- **Project-level (shared)** — available across all services in the project
+- **Service-level** — specific to a single service
+
+## Project-Level Variables
+
+### Practical Example
 
 Let's consider a common scenario where you have:
 - A PostgreSQL database
@@ -41,11 +49,28 @@ DATABASE_URL=${{project.DATABASE_URL}}
   Dokploy will automatically replace `${{project.DATABASE_URL}}` with the value defined in the project's shared variables.
 </Callout>
 
-## Best Practices
+You can use shared environment variables in all the services available in dokploy.
+
+## Service-Level Variables
+
+Service-level variables are specific to a single service and can be used to override shared variables or define service-specific configurations.
+
+### Practical Example
+
+Let's say you have a service that requires a different database user. You can define a service-level variable:
+
+```bash
+DATABASE_USER=service_user
+DATABASE_URL=postgresql://${{DATABASE_USER}}:${{project.DATABASE_PASSWORD}}@service-database:5432/postgres
+```
+
+<Callout type="info">
+  Preview Deployments environments also include a service-level variable called `DOKPLOY_DEPLOY_URL`, which points to the deployment URL of the service.
+  It can be used as `${{DOKPLOY_DEPLOY_URL}}` for variables like `APP_URL=https://${{DOKPLOY_DEPLOY_URL}}`.
+</Callout>
+
+### Best Practices
 
 - Use shared variables for credentials and configurations that repeat across services
 - Keep descriptive variable names
-- Document the purpose of each variable for easier maintenance
+- Document the purpose of each variable for easier maintenance
-
-
-You can use shared enviroment variables in all the services available in dokploy.