Added demo multiple compose files

Author: JaimeSalas

01-contenedores/contenedores-vi/docker-compose-extension/demos/03-use-multiple-compose-files.md

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+# Use Multiple Compose files
+
+Using multiple Compose files lets you customize a Compose application for different environments or workflows. This is useful for large applications that may use dozens of containers, with ownership distributed across multiple teams. For example, if your organization or team uses a monorepo, each team may have their own “local” Compose file to run a subset of the application. They then need to rely on other teams to provide a reference Compose file that defines the expected way to run their own subset. Complexity moves from the code in to the infrastructure and the configuration file.
+
+The quickest [merge](https://docs.docker.com/compose/how-tos/multiple-compose-files/merge/). However, [merging rules](https://docs.docker.com/compose/how-tos/multiple-compose-files/merge/#merging-rules) means this can soon get quite complicated.
+
+Create `compose.yml`
+
+```yml
+services:
+  web:
+    image: nginx
+    depends_on:
+      - db
+      - cache
+  
+  db:
+    image: postgres:latest
+
+  cache:
+    image: redis:latest
+```
+
+Create `compose.override.yml`
+
+```yml
+services:
+  web:
+    volumes:
+      - 'code:/tmp'
+    ports:
+      - 8900:80
+    environment:
+      DEBUG: 'true'
+
+  db:
+    ports:
+      - 5432:5432
+    environment:
+      POSTGRES_PASSWORD: password
+      POSTGRES_USER: admin
+
+  cache:
+    ports:
+      - 6379:6379
+
+volumes:
+  code:
+
+```
+
+```bash
+docker compose up
+```
+
+Now reads by dedfault `compose.override` and applies the updates. Let's say that we want to run in another environment:
+
+Create `compose.staging.yml`
+
+```yml
+services:
+  web:
+    ports:
+      - 80:80
+    environment:
+      PRODUCTION: 'true'
+
+  db:
+    environment:
+      POSTGRES_HOST_AUTH_METHOD: trust
+  
+  cache:
+    environment:
+      TTL: '500'
+```
+
+```bash
+docker compose -f compose.yml -f compose.staging.yml up
+```