Clarifies compose push destination and requirements (#2843)

* Clarifies compose push destination and requirements

* Edits

compose/reference/push.md

@@ -12,4 +12,24 @@ Options:
     --ignore-push-failures  Push what it can and ignores images with push failures.
 ```
 
-Pushes images for services.
+Pushes images for services to their respective `registry/repository`.
+
+The following assumptions are made:
+
+- You are pushing an image you have built locally
+
+- You have access to the build key
+
+## Example
+
+```yaml
+version: '3'
+services:
+  service1:
+    build: .
+    image: localhost:5000/yourimage  # goes to local registry
+
+  service2:
+    build: .
+    image: youruser/yourimage  # goes to youruser DockerHub registry
+```