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/Dockerfile.node-runner

@@ -0,0 +1,12 @@
+# Base container for npm-based MCP servers (STDIO transport).
+# mcpd uses this image to run `npx -y <packageName>` when a server
+# has packageName but no dockerImage.
+FROM node:20-alpine
+
+WORKDIR /mcp
+
+# Pre-warm npx cache directory
+RUN mkdir -p /root/.npm
+
+# Default entrypoint — overridden by mcpd via container command
+ENTRYPOINT ["npx", "-y"]