feat(ui): support serving under a subpath via MCP_REGISTRY_UI_BASE_PATH

[nejch]

Motivation and Context

#757 added a simple UI, and although I understand this was a simple example and most production downstream registries will have a custom UI, I like the idea of starting small and staying consistent with https://registry.modelcontextprotocol.io/.

In my case this is at https://api.company.com/mcp/registry behind a reverse proxy, but the UI redirects and expects to be served at the root, so the UI is broken.

This PR adds subpath handling to the browser side template but does not adjust routing itself (this is normally handled by the reverse proxy).

If this is accepted I'd add a few more adjustments as a follow-up, so that downstream registries can use the same UI but a few custom options like the title and so on.

How Has This Been Tested?

docker compose + nginx

Dockerfile.dev

FROM golang:1.26-alpine AS build
WORKDIR /src
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN go build -o /registry ./cmd/registry
FROM alpine:3
COPY --from=build /registry /registry
ENTRYPOINT ["/registry"]

nginx.conf

server {
    listen 9090;
    location /mcp/registry/ {
        proxy_pass http://registry:8080/;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }
    location = /mcp/registry {
        return 301 /mcp/registry/;
    }
}

docker-compose.dev.yml

services:
  registry:
    build:
      context: .
      dockerfile: Dockerfile.dev
    container_name: registry
    depends_on:
      postgres:
        condition: service_healthy
    environment:
      - MCP_REGISTRY_DATABASE_URL=postgres://mcpregistry:mcpregistry@postgres:5432/mcp-registry
      - MCP_REGISTRY_ENABLE_REGISTRY_VALIDATION=false
      - MCP_REGISTRY_SEED_FROM=data/seed.json
      - MCP_REGISTRY_JWT_PRIVATE_KEY=8103179d8ef955f6d3de6d6217224a909ec4060529dfeb1d4ca5a994537658cd
      - MCP_REGISTRY_ENABLE_ANONYMOUS_AUTH=true
      - MCP_REGISTRY_UI_BASE_PATH=/mcp/registry
    volumes:
      - ./data:/data:ro
    restart: "unless-stopped"
  postgres:
    image: postgres:16-alpine
    container_name: postgres
    environment:
      POSTGRES_DB: mcp-registry
      POSTGRES_USER: mcpregistry
      POSTGRES_PASSWORD: mcpregistry
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U mcpregistry -d mcp-registry"]
      interval: 1s
      retries: 30
    restart: "unless-stopped"
  nginx:
    image: nginx:alpine
    container_name: nginx
    depends_on:
      - registry
    ports:
      - "9090:9090"
    volumes:
      - ./nginx.conf:/etc/nginx/conf.d/default.conf:ro
    restart: "unless-stopped"

docker compose -f docker-compose.dev.yml up --build

• Browse to http://localhost:9090/mcp/registry
• API Reference link points to /mcp/registry/docs
• Servers load correctly (API calls go through /mcp/registry/v0.1/servers)
• Browser navigation/history stays under /mcp/registry

Also tested with /my-registry as the base path and without a path to confirm it works with arbitrary prefixes.

Breaking Changes

Shouldn't be breaking anything, but see testing example above.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[nejch]

If this PR is accepted I could change the custom templating here in a follow-up to html/template or so to also accept things like custom titles and so on.

This was just a quick and minimal change.

[GGOemea]

ok