docs: Clarify environment variable injection options for Docker Compose

- Updated the documentation to specify two methods for injecting environment variables into containers: using the `env_file` option to load all variables or referencing specific variables with the `${VAR_NAME}` syntax.

apps/docs/content/docs/core/docker-compose/index.mdx

@@ -23,7 +23,9 @@ Configure the source of your code, the way your application is built, and also m
 The code editor in Dokploy allows you to define environment variables for your Docker Compose deployment. By default, Dokploy saves these variables to a `.env` file in the same directory as your `docker-compose.yml`.
 
 <Callout type="warning">
-Environment variables set in the UI are written to the `.env` file, but are **not automatically injected into containers**. To pass variables into a service, use the `env_file` option in your `docker-compose.yml`:
+Environment variables set in the UI are written to the `.env` file, but are **not automatically injected into containers**. You have two options:
+
+**1. Inject all variables** — Use the `env_file` option in your `docker-compose.yml` to load every variable from the `.env` file into the container:
 
 ```yaml
 services:
@@ -31,6 +33,16 @@ services:
     env_file:
       - .env
 ```
+
+**2. Use specific variables** — Reference only the variables you need using the standard `${VAR_NAME}` syntax in your `docker-compose.yml`:
+
+```yaml
+services:
+  app:
+    environment:
+      - DATABASE_URL=${DATABASE_URL}
+      - API_KEY=${API_KEY}
+```
 </Callout>
 
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