Update docker-images.md

Author: deliahu

docs/workloads/dependencies/docker-images.md

@@ -1,6 +1,6 @@
 # Docker images
 
-You can build a custom Docker image for use in your APIs, e.g. to avoid installing dependencies during replica initialization or to have smaller images.
+You can build a custom Docker image for use in your APIs. Common reasons to do this are to avoid installing dependencies during replica initialization, to have smaller images, and/or to mirror images to your cloud's container registry (for speed and reliability).
 
 ## Create a Dockerfile