feat: add node-runner base image for npm-based MCP servers

STDIO servers with packageName (e.g. @leval/mcp-grafana) need a Node.js
container that runs `npx -y <package>`. Previously, packageName was used
as a Docker image reference causing "invalid reference format" errors.

- Add Dockerfile.node-runner: minimal node:20-alpine with npx entrypoint
- Update instance.service.ts: detect npm-based servers and use node-runner
  image with npx command instead of treating packageName as image name
- Fix NanoCPUs: only set when explicitly provided (kernel CFS not available
  on all hosts)
- Add mcp-servers network with explicit name for container isolation
- Configure MCPD_NODE_RUNNER_IMAGE and MCPD_MCP_NETWORK env vars

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

deploy/docker-compose.yml

@@ -30,6 +30,8 @@ services:
       MCPD_PORT: "3100"
       MCPD_HOST: "0.0.0.0"
       MCPD_LOG_LEVEL: info
+      MCPD_NODE_RUNNER_IMAGE: mcpctl-node-runner:latest
+      MCPD_MCP_NETWORK: mcp-servers
     depends_on:
       postgres:
         condition: service_healthy
@@ -48,6 +50,16 @@ services:
       retries: 3
       start_period: 10s
 
+  # Base image for npm-based MCP servers (built once, used by mcpd)
+  node-runner:
+    build:
+      context: ..
+      dockerfile: deploy/Dockerfile.node-runner
+    image: mcpctl-node-runner:latest
+    profiles:
+      - build
+    entrypoint: ["echo", "Image built successfully"]
+
   postgres-test:
     image: postgres:16-alpine
     container_name: mcpctl-postgres-test
@@ -71,8 +83,11 @@ networks:
   mcpctl:
     driver: bridge
   mcp-servers:
+    name: mcp-servers
     driver: bridge
-    internal: true
+    # Not internal — MCP servers need outbound access to reach external APIs
+    # (e.g., Grafana, Home Assistant). Isolation is enforced by not binding
+    # host ports on MCP server containers; only mcpd can reach them.
 
 volumes:
   mcpctl-pgdata: