introduce service.external

Signed-off-by: Nicolas De Loof <[email protected]>

Author: ndeloof

spec.md

@@ -1031,6 +1031,28 @@ duplicates resulting from the merge are not removed.
 
 Any other allowed keys in the service definition should be treated as scalars.
 
+### external
+
+If set to `true`, `external` specifies that this service lifecycle is maintained outside of that application defined
+this the Compose file. Compose Implementations SHOULD NOT attempt to create container(s) for this service.
+
+In the example below, `db` is a database service managed on a cloud infrastructure. Instead of attempting to create 
+this service as a container, Compose implementations SHOULD consider this service as available for dependent services.
+
+This attribute is expected to be used in a Compose override file to replace a service definition with an external resource.
+
+```yaml
+services:
+  app:
+    image: mycompay/app
+    depends_on:
+      - db
+  db:
+    external: true
+    container_name: database.cloud.provider
+```
+
+
 ### external_links
 
 `external_links` link service containers to services managed outside this Compose application.