Fix pushing images to local registry documentation

Adding -e REGISTRY_PROXY_REMOTEURL=https://registry-1.docker.io to the
docker run command in the start-registry script configures the local
registry as a pull through cache. However, pushing of images to the
local registry fails.

Pushing to a registry configured as a pull-through cache is unsupported.
For more information, reference the proxy section in Docker
Documentation (https://docs.docker.com/registry/configuration/)

The start_registry script has been updated in the patch set
https://review.openstack.org/#/c/426338/

Change-Id: If62b40d8316af7c3593523493cad8a904d35dceb