Make deployment registry URL configurable

[arjunkomath]

Summary

• read the web service registry URL from REGISTRY_URL
• fall back to the bundled registry:5000 endpoint
• add the default to example and installer-generated environment configuration
• keep the registry URL out of the migration service environment

Validation

• git diff --check
• bash -n deployment/install.sh

Docker Compose config validation was not run because Docker is unavailable in the orb.