Pre-install packages at build time for network isolation

[JAORMX]

Summary

This PR fixes #1597 by modifying container templates to pre-install packages at build time rather than runtime, enabling MCP servers to run in network-isolated environments.

Problem

MCP servers currently require network access at runtime to download packages, which prevents them from running in network-isolated environments. This poses security concerns and limits deployment options in restricted environments.

Solution

Implemented multi-stage Docker builds for all three package managers (NPX, UVX, Go) to pre-install packages and their dependencies during the build phase.

Changes

NPX Template

• Added multi-stage build with FROM node:22-alpine AS builder
• Pre-installs npm packages using npm install --save in builder stage
• Runtime uses npx --no-install to prevent network access
• Copies node_modules to runtime stage

UVX Template

• Added multi-stage build with FROM python:3.13-slim AS builder
• Uses uv tool install to properly install Python tools with their executables
• Handles executable discovery automatically
• Copies the tool installation to runtime stage

Go Template

• Added multi-stage build with FROM golang:1.25-alpine AS builder
• Pre-installs Go packages with go install handling version specifiers
• Copies compiled binary to minimal Alpine runtime image

Tests

• Updated templates_test.go to match new multi-stage build structure
• All tests pass successfully

Testing

Tested all three templates with real MCP servers using the --isolate-network flag:

• ✅ NPX: @upstash/context7-mcp
• ✅ UVX: mcp-server-browser-use
• ✅ Go: github.com/StacklokLabs/osv-mcp/cmd/server

All servers started successfully and responded to MCP protocol messages without network access.

Impact

• Enables MCP servers to run in air-gapped environments
• Improves security by eliminating runtime network dependencies
• Maintains backward compatibility with existing configurations
• All package managers now support the --isolate-network flag

Checklist

• Code changes
• Tests updated and passing
• Manual testing with network isolation
• Documentation (templates are self-documenting)