Support dual Docker registry environment variables for Kubernetes deployments

[Copilot]

This PR adds support for separate Docker registry URLs to enable efficient Kubernetes deployments where internal cluster communication can use ClusterIP while external clients receive publicly accessible registry URLs.

Problem

In Kubernetes deployments, the catalog service needed to:

1. Push/pull Docker images efficiently using internal cluster networking (ClusterIP)
2. Provide external clients with publicly accessible registry URLs for image references

Previously, only a single docker_registry_host environment variable was supported, forcing a choice between internal efficiency and external accessibility.

Solution

Added support for a second environment variable docker_registry_client_host that works alongside the existing docker_registry_host:

• docker_registry_host - Used for internal Docker operations (push/pull) within the cluster
• docker_registry_client_host - Used for client-facing image names returned via API calls

Configuration Example

# Internal cluster registry for efficient operations
export docker_registry_host="internal-registry.cluster.local:5000"
# External registry URL accessible to clients  
export docker_registry_client_host="registry.example.com"

Backward Compatibility

When only docker_registry_host is configured, it serves both purposes, maintaining full backward compatibility with existing deployments.

Changes Made

• Updated deployment configuration template to support the new environment variable
• Modified controller to handle dual registry configuration with fallback logic
• Updated registrar to use appropriate registry for each operation type
• Added comprehensive unit and integration tests
• Updated documentation and examples

The implementation ensures that Docker images are built and tagged with client-accessible names while push operations use the most efficient internal network path.

Fixes #175.

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