🐳 Release: Docker build stabilization for beta

[jongalloway]

Fix docker build restore failures and validate runnable demo container.

Tasks

• Ensure Directory.Packages.props and NuGet.Config are available before dotnet restore in Dockerfile
• Validate restore with pinned versions; fix NU110x/NU160x
• Smoke test container startup (env vars and config)
• Publish image to GHCR as part of release

Acceptance Criteria

• docker build completes on clean machine
• docker run starts demo and responds to a basic health/check endpoint

Parent: #76