docs: address issue #24158 (#24159)

## Summary
Changed `build: alpine` to `build: ./alpine` in extends examples to
clarify it's a directory path, not an image name.

## Changes
- Updated two examples in extends documentation to use `build: ./alpine`
instead of `build: alpine` to avoid confusion with the Alpine Docker
image

Fixes #24158

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content/manuals/compose/how-tos/multiple-compose-files/extends.md

@@ -66,7 +66,7 @@ To include the service `webapp` in the final project when extending services fro
 ```yaml
 services:
   web:
-    build: alpine
+    build: ./alpine
     command: echo
     extends:
       file: common-services.yml
@@ -86,7 +86,7 @@ If you define services in the same Compose file and extend one service from anot
 ```yaml 
 services:
   web:
-    build: alpine
+    build: ./alpine
     extends: webapp
   webapp:
     environment: