Support for Custom MCP Registry URL via Environment Variable

[karthicksndr]

Problem:

Currently, the MCP Registry UI is hardcoded to load from https://registry.modelcontextprotocol.io/. In enterprise environments, air-gapped setups, or private local deployments, users may need to host their own registry or a mirrored version of the UI. There is currently no straightforward way to redirect the UI to a custom source without modifying the source code.

Request:

Allow MCP_REGISTRY_CUSTOM_MCP_REGISTRY_URL as a docker env parameter

[Mr-w-22]