fix(docker): use ASP.NET runtime and optimize build + update README

[gsayer]

Summary

This commit fixes Docker build issues and improves usability when running PayrollEngine.PayrollConsole in containers.

Issue

$ docker run -it --rm payroll-console Setup.pecmd
You must install or update .NET to run this application.

App: /app/PayrollEngine.PayrollConsole.dll
Architecture: arm64
Framework: 'Microsoft.AspNetCore.App', version '9.0.0' (arm64)
.NET location: /usr/share/dotnet/

No frameworks were found.

Learn more:
https://aka.ms/dotnet/app-launch-failed

To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.AspNetCore.App&framework_version=9.0.0&arch=arm64&rid=linux-arm64&os=debian.12

Changes

Dockerfile

• Use mcr.microsoft.com/dotnet/aspnet:9.0 for runtime stage
• Multi-stage build with cached restore and publish
• Add --no-restore to publish for faster, reproducible builds

README > Docker

• Show how to pass PayrollApiConnection
• Show how to mount volumes for .pecmd scripts and sample data