[What's New Analysis] Document container-based builds / WithDockerfileBuilder

Summary

Aspire 13.0 introduced automatic Dockerfile generation and WithDockerfileBuilder APIs for Python, JavaScript, and custom scenarios. These are experimental but not documented.

What's Missing

From what's-new-in-aspire-13:

Automatic Dockerfile Generation

Python and JavaScript apps automatically generate production-ready Dockerfiles when publishing:

• Multi-stage builds
• Package manager detection (pip/uv, npm/yarn/pnpm)
• Python version detection from .python-version, pyproject.toml
• Node.js version detection from .nvmrc, .node-version, package.json

WithDockerfileBuilder (Experimental)

// Experimental API
#pragma warning disable ASPIREXXX
var app = builder.AddContainer("myapp", "myimage")
    .WithDockerfileBuilder(...);
#pragma warning restore ASPIREXXX

Suggested Content

1. Auto Dockerfile generation - How it works for Python/JS
2. Version detection - Where versions are read from
3. Customizing builds - WithDockerfileBuilder experimental API
4. Multi-stage builds - How they're structured
5. Examples - Python with uv, Node with pnpm